Being conceited is useless (4)
I believe the answer is:
vain
I believe this clue is a double definition.
'being conceited' is the first definition.
('vain' can be a synonym of 'conceited')
'useless' is the second definition.
(I've seen this before)
'is' is the link.
(Other definitions for vain that I've seen before include "Egotistical" , "Conceited or unsuccessful" , "Conceited - or a waste of time" , "Having too much pride in one's appearance" , "Conceited or unavailing" .)