Sheep in activewear (3)
I believe the answer is:
ewe
'sheep' is the definition.
(a ewe is a female sheep)
'in activewear' is the wordplay.
'in' indicates the answer is hidden within the clue.
'EWE' is hidden in the letters of 'activewear'.
(Other definitions for ewe that I've seen before include "Lamb supplier" , "Partner to ram" , "Ram's partner?" , "One looking sheepish" , "Source of milk" .)