Needs to become thick (5)
I believe the answer is:
dense
'thick' is the definition.
(similar in meaning)
'needs to become' is the wordplay.
'to become' indicates an anagram.
'needs' is an anagram of 'DENSE'.
(Other definitions for dense that I've seen before include "Solid, thick" , "Thick; stupid" , "Close-knit" , "Heavy" , "Closely crowded together" .)