Work interrupted by a sport (4)
I believe the answer is:
play
'sport' is the definition.
('play' can be a synonym of 'sport')
'work interrupted by a' is the wordplay.
'work' becomes 'ply' (I've seen this before**).
'interrupted by' means one lot of letters goes inside another.
'ply' placed around 'a' is 'PLAY'.
(Other definitions for play that I've seen before include "Freedom of movement" , "Have fun -- theatrical work" , "Frolic" , "Theatrical production" , "Dramatic work" .)