Pain reaches inner part (4)
I believe the answer is:
ache
'pain' is the definition.
(ache is a kind of pain)
'reaches inner part' is the wordplay.
'inner part' indicates the answer is hidden within the clue (I've seen 'in part' mean this).
'ACHE' is hidden in the letters of 'reaches'.
(Other definitions for ache that I've seen before include "Longing" , "anguish" , "Nagging pain" , "Long ? pain" , "painful" .)