Needs to turn thick (5)
I believe the answer is:
dense
'thick' is the definition.
(I know that thick can be written as dense)
'needs to turn' is the wordplay.
'to turn' is an anagram indicator (letters turn into something else).
'needs' is an anagram of 'DENSE'.
(Other definitions for dense that I've seen before include "Close-packed" , "Stupid, crowded together" , "Obtuse - impenetrable" , "Thick, impenetrable" , "Closely-spaced" .)