Useless to be conceited (4)
I believe the answer is:
vain
I believe this clue is a double definition.
'useless' is the first definition.
(I've seen this in another clue)
'be conceited' is the second definition.
(synonyms)
'to' is the link.
(Other definitions for vain that I've seen before include "Conceited, self-important" , "Useless; conceited" , "Conceited, arrogant" , "Conceited - or hopeless" , "thoughtless" .)