The female holding the ball may be the trainer (4)
I believe the answer is:
shoe
'the trainer' is the definition.
(I've seen this before)
'the female holding the ball' is the wordplay.
'the female' becomes 'she' (I've seen this before**).
'holding' means one lot of letters goes inside another.
'the ball' becomes 'o'.
'she' placed around 'o' is 'SHOE'.
'may be' is the link.
(Other definitions for shoe that I've seen before include "Camera mounting" , "From which playing cards are dealt one at a time" , "Brogue, for example" , "Where a dealer stashes a pack" , "Card dispenser" .)