Hard in diet to get supple (5)
I believe the answer is:
lithe
'supple' is the definition.
('lithe' can be a synonym of 'supple')
'hard in diet' is the wordplay.
'hard' becomes 'h'.
'in' indicates putting letters inside.
'diet' becomes 'lite' ('lite' can be a synonym of 'diet').
'h' going inside 'lite' is 'LITHE'.
'to get' is the link.
(Other definitions for lithe that I've seen before include "Lissom, supple" , "quick to move" , "Physically supple, agile" , "Physically flexible or supple" , "grace" .)