Possibly needs to be thick (5)
I believe the answer is:
dense
'thick' is the definition.
(similar in meaning)
'possibly needs' is the wordplay.
'possibly' indicates anagramming the letters.
'needs' with letters rearranged gives 'DENSE'.
'to be' is the link.
(Other definitions for dense that I've seen before include "Closely compacted" , "Thick, impenetrable" , "Stupid; compacted" , "Obtuse - impenetrable" , "Compressed - obtuse" .)