Possibly needs to be thick (5)
I believe the answer is:
dense
'thick' is the definition.
('dense' can be a synonym of 'thick')
'possibly needs' is the wordplay.
'possibly' indicates an anagram.
'needs' is an anagram of 'DENSE'.
'to be' acts as a link.
(Other definitions for dense that I've seen before include "Closely compacted" , "Thick, impenetrable" , "Stupid; compacted" , "Obtuse - impenetrable" , "Compressed - obtuse" .)