Needs to turn compact (5)
I believe the answer is:
dense
'compact' is the definition.
(synonyms)
'needs to turn' is the wordplay.
'to turn' indicates an anagram (letters turn into something else).
'needs' anagrammed gives 'DENSE'.
(Other definitions for dense that I've seen before include "Obtuse - impenetrable" , "Close - compact - stupid" , "Needs (anag) -- thick" , "Closely crowded together" , "Maybe bovine" .)