Pursue trimness, concealing fat (4)
I believe the answer is:
suet
'fat' is the definition.
(I know that suet is a type of fat)
'pursue trimness concealing' is the wordplay.
'concealing' indicates a hidden word.
'SUET' can be found hidden inside 'pursue trimness'.
(Other definitions for suet that I've seen before include "Fat used in foods" , "Hard fat in cattle and sheep" , "Hard animal fat used in cooking" , "Hard white cooking fat" , "Hard fat from beef and sheep" .)