Closely packed (5)
(Other definitions for compact that I've seen before include "cosmetics case" , "Mutual agreement" , "bargain" , "woman's case" , "Tightly grouped" .)
(Other definitions for crowded that I've seen before include "Jam-packed" , "With too many people" , "Packed (of people)" , "With many present" , "Thronged, overfilled" .)
(Other definitions for serried that I've seen before include "standing close together" , "Close together in rows" , "in packs" , "Rows pressed together, military ranks say" , "(Ranks) in close formation" .)
'closely packed' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dense that I've seen before include "Closely-spaced" , "Closely spaced or extremely stupid" , "Maybe bovine" , "Heavy" , "Solid, thick" .)