Needs arrangement to be tightly packed (5)
I believe the answer is:
dense
'be tightly packed' is the definition.
(I've seen this before)
'needs arrangement' is the wordplay.
'arrangement' indicates anagramming the letters.
'needs' is an anagram of 'DENSE'.
'to' is the link.
(Other definitions for dense that I've seen before include "Obtuse - impenetrable" , "Close-knit" , "Thick, impenetrable" , "Tightly packed - stupid" , "Stupid, crowded together" .)