Go in some pain (5)
I believe the answer is:
agony
'pain' is the definition.
(agony is a kind of pain)
'go in some' is the wordplay.
'in' means one lot of letters goes inside another.
'some' becomes 'any' (similar in meaning).
'go' going into 'any' is 'AGONY'.
(Other definitions for agony that I've seen before include "Excruciating pain" , "Intense pain" , "Extreme suffering - kind of aunt" , "Great pain or suffering" , "Mental suffering" .)