They validate expression of pain in short poem (8)
I believe the answer is:
vouchers
'they validate' is the definition.
(things which vouch for people or validate them)
'expression of pain in short poem' is the wordplay.
'expression of pain' becomes 'ouch' (I've seen this before).
'in' is an insertion indicator.
'short' means to remove the last letter.
'poem' becomes 'verse' (verse is a kind of poem).
'verse' with its final letter removed is 'vers'.
'ouch' placed within 'vers' is 'VOUCHERS'.
(Other definitions for vouchers that I've seen before include "Coupons" , "Receipts" , "Meal tickets?" , "Certificates" .)