Swarm left on a crop (5)
I believe the answer is:
crawl
'swarm' is the definition.
(I know that crawl is a more specific form of swarm)
'left on a crop' is the wordplay.
'left' becomes 'l' (common abbreviation).
'on' is a charade indicator (letters next to each other).
'a crop' becomes 'craw' (synonyms).
'l' put after 'craw' is 'CRAWL'.
(Other definitions for crawl that I've seen before include "Suck up to" , "Make no haste" , "Move as snakes do, or on hands and knees" , "Creep or move slowly" , "Go on hands and knees" .)