Boast of credit that hurt (4)
I believe the answer is:
crow
'boast' is the definition.
(to crow about something is to boast about it)
'credit that hurt' is the wordplay.
'credit' becomes 'cr'.
'that hurt' becomes 'ow' (exclamation of pain).
'cr'+'ow'='CROW'
'of' is the link.
(Other definitions for crow that I've seen before include "one often getting stoned!" , "bar" , "Boast, swagger, triumph" , "Black bird; boast" , "Express gleeful triumph" .)