To gather, crowd returned (4)
I believe the answer is:
meet
'to gather' is the definition.
(synonyms)
'crowd returned' is the wordplay.
'crowd' becomes 'teem'.
'returned' shows that the letters should be reversed in order.
'teem' backwards is 'MEET'.
(Other definitions for meet that I've seen before include "assembly at hunt" , "Touch; suitable (arch.)" , "Join" , "Countryside assembly now perhaps not legal" , "Hunting gathering" .)