Wood popular in gym (4)
I believe the answer is:
pine
'wood' is the definition.
(pine is a kind of wood)
'popular in gym' is the wordplay.
'popular' becomes 'in'.
'in' indicates putting letters inside.
'gym' becomes 'pe' (**).
'in' put into 'pe' is 'PINE'.
(Other definitions for pine that I've seen before include "type of wood" , "A tree to grieve for" , "Scots fir" , "Tree, waste away" , "Mope; tree" .)