Thickly packed (5)
I believe the answer is:
dense
'thickly packed' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for dense that I've seen before include "Needs (anag) -- thick" , "Slow-witted" , "Closely crowded together" , "Thick, impenetrable" , "Solid, thick" .)