Favourite to admit retail doesn't start ball (6)
I believe the answer is:
pellet
'ball' is the definition.
(I know that pellet is a type of ball)
'favourite to admit retail doesn't start' is the wordplay.
'favourite' becomes 'pet' (synonyms).
'to admit' is an insertion indicator.
'retail' becomes 'sell retail' (synonyms).
'doesn't start' means to remove the first letter.
'sell' with its initial letter taken off is 'ell'.
'pet' placed around 'ell' is 'PELLET'.
(Other definitions for pellet that I've seen before include "Small piece of buckshot" , "Small bullet" , "Small ball of shot" , "Piece of shot for gun" , "Small round ball" .)