Poster vacant doofus puts together (4)
I believe the answer is:
adds
'puts together' is the definition.
(I've seen this in another clue)
'poster vacant doofus' is the wordplay.
'poster' becomes 'ad' (short for advert).
'vacant' says to hollow out the word (remove centre letters) (a vacant or emptied out form of the word).
'doofus' with its centre removed is 'ds'.
'ad'+'ds'='ADDS'
(Other definitions for adds that I've seen before include "Tots up" , "Joins to something else" , "Reckons" , "Puts in; tacks on" , "Goes on to say" .)