Being thick, needs to change (5)
I believe the answer is:
dense
'being thick' is the definition.
('dense' can be similar in meaning to 'thick')
'needs to change' is the wordplay.
'to change' indicates anagramming the letters.
'needs' with letters rearranged gives 'DENSE'.
(Other definitions for dense that I've seen before include "Closely packed" , "Close knit" , "Close-knit" , "Compressed - obtuse" , "Thick, compact or crowded" .)