Certify victory with a pained expression (5)
I believe the answer is:
vouch
'certify' is the definition.
(I know that certify is a kind of the vouch)
'victory with a pained expression' is the wordplay.
'victory' becomes 'v'.
'with' says to put letters next to each other.
'a pained expression' becomes 'ouch' (I've seen this before).
'v'+'ouch'='VOUCH'
(Other definitions for vouch that I've seen before include "Give personal assurance; guarantee" , "Be surety (for)" , "Declare support" , "Testify" , "warrant" .)