Exploit energy retained by adipose tissue (4)
I believe the answer is:
feat
'exploit' is the definition.
('feat' can be a synonym of 'exploit')
'energy retained by adipose tissue' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mca).
'retained by' means one lot of letters goes inside another.
'adipose tissue' becomes 'fat' ('fat' can be a synonym of 'adipose tissue'**).
'e' put into 'fat' is 'FEAT'.
(Other definitions for feat that I've seen before include "Achievement needing great courage or skill" , "Great deed" , "Notable act" , "Deed; achievement" , "Exploit; achievement" .)