Blues boss Danny Cowley has identified the 22-year-old as someone he wants to add to his attacking ranks.
And efforts are ongoing in a bid to make the move happen before the transfer window closes on September 1.
Cowley has overhauled his forward options this summer with the permanent signing of Colby Bishop and the loan arrivals of Joe Pigott and Dane Scarlett.
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A deal for Blackpool winger Owen Dale has also been done, although is yet to be officially confirmed by the club.
Meanwhile, the Pompey manager also sanctioned the sale of Marcus Harness to Ipswich for an undisclosed fee last month.
But a desire to add another permanent member of that group has been something Cowley has been eager to do this window as well.
And with the Blues specifically looking to add youth, pace, power and athleticism into the mix, Mason-Clarke has been identified to do exactly that.
Despite their £500,000 move for Bishop, Pompey have money to spare following Harness’ switch to the Tractor Boys and the departures of high-earners during Cowley’s first full season in charge.
A fee is yet to be agreed with Barnet for a player who came through their youth ranks.
Yet the Blues are determined to get a deal done so that Cowley and his coaching team can start working with the emerging talent.
Mason-Clarke impressed for the Bees in their recent pre-season friendly against the Blues at The Hive last month.
Featuring in the first half, he gave the Pompey defence plenty to think about and operated in various forward roles, which is another attractive quality he can bring to the table.
The Blues’ interest, though, is a long-standing one, with the forward continuing his development at an impressive rate in recent seasons.
Making his Barnet debut as a 17-year-old in 2016, he has gone on to make 176 appearances for the National League side, scoring 17 goals.
Last term was his most prolific in front of goal, with Mason-Clark scoring six goals in 39 league appearances.
Barnet kick off their National League campaign today with a home game against Halifax.