“A Greek”, this leading chap? (5,4)
I believe the answer is:
'a greek this leading chap?' is the definition.
'alpha male' can be an answer for 'leading chap?' (important or powerful male). I'm unsure of the remainder of the definition.
'a greek this leading chap?' is the wordplay.
'a greek' becomes 'alpha' (Greek equivalent to 'A').
'this leading' means one lot of letters go next to another (some letters go in front of others).
'chap?' becomes 'male' (chap can mean a man).