In discomfort or mental anguish (7)
I believe the answer is:
torment
'anguish' is the definition.
(tormenting is a kind of anguishing)
'in discomfort or mental' is the wordplay.
'in' says the answer is hidden in the clue.
'TORMENT' is hidden within 'discomfort or mental'.
(Other definitions for torment that I've seen before include "Cause great pain to" , "Hound" , "More TNT (anag.)" , "Harass" , "Pester annoyingly" .)