Old hat has function as cap (5)
I believe the answer is:
outdo
'cap' is the definition.
(I've seen this before)
'old hat has function' is the wordplay.
'old hat' becomes 'out' (both can mean old-fashioned or outdated).
'has' is a charade indicator (letters next to each other).
'function' becomes 'do' (both can mean an event).
'out'+'do'='OUTDO'
'as' is the link.
(Other definitions for outdo that I've seen before include "Cap" , "Surpass someone at something" , "Perform better than surpass" , "Br superior to (someone)" , "Eclipse" .)