Mug bishop during play (4)
I believe the answer is:
toby
'mug' is the definition.
(toby is a kind of mug)
'bishop during play' is the wordplay.
'bishop' becomes 'b' (abbreviation used in chess).
'during' means one lot of letters goes inside another.
'play' becomes 'toy' ('toy' can be a synonym of 'play').
'b' going within 'toy' is 'TOBY'.
(Other definitions for toby that I've seen before include "Kind of jug" , "This Jug is like man with three-cornered hat" , "Mr Punch's dog, beer-jug" , "Belch, perhaps" , "Member of the 12 [SHANDY] family" .)