Opening gambit's beginning with a pawn (3)
I believe the answer is:
gap
'opening' is the definition.
(gap is a kind of opening)
'gambit's beginning with a pawn' is the wordplay.
'beginning' says to take the initial letters.
'with' is a charade indicator (letters next to each other).
'pawn' becomes 'p'.
The first letter of 'gambit' is 'g'.
'g'+'a'+'p'='GAP'
(Other definitions for gap that I've seen before include "Break, hole, space" , "Breach or lacuna" , "Hiatus" , "A breach or pass" , "prospective students may enjoy this year?" .)