Withstanding? Is inside relaxing (9)
I believe the answer is:
resisting
'withstanding?' is the definition.
('resist' can be a synonym of 'withstand')
'is inside relaxing' is the wordplay.
'inside' indicates putting letters inside.
'relaxing' becomes 'resting' (I've seen this in another clue).
'is' placed within 'resting' is 'RESISTING'.
(Other definitions for resisting that I've seen before include "smart, that!" , "Withstanding" , "Refraining from" .)