Part of stomach experiencing pain (4)
I believe the answer is:
ache
'pain' is the definition.
(ache is a kind of pain)
'part of stomach experiencing' is the wordplay.
'part of' indicates a hidden word.
'ACHE' is hidden in the letters of 'stomach experiencing'.
(Other definitions for ache that I've seen before include "Long ? pain" , "Dull continuous pain" , "Dull throbbing pain" , "suffering" , "yearning" .)