Determined expression of girlfriend tackling a Brockwell Genius (4,4)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

GF (abbreviation for girlfriend), containing (tackling) A + ME (Brockwell, our crossword setter), then ACE (genius = expert). A determined expression adopted to intimidate an opponent during a sports contest, or more generally while concentrating hard on something.