Only lose break (4)
I believe the answer is:
sole
'only' is the definition.
(I know that only can be written as sole)
'lose break' is the wordplay.
'break' indicates an anagram.
'lose' anagrammed gives 'SOLE'.
(Other definitions for sole that I've seen before include "A lone sort of fish - lose it" , "European flatfish" , "Flatfish; exclusive" , "fish in the sea!" , "fish > that's never less than foot in length" .)