Come across swarm on the way back (4)
I believe the answer is:
meet
'come across' is the definition.
('meet' can be a synonym of 'come across')
'swarm on the way back' is the wordplay.
'swarm' becomes 'teem' ('teem' can be a synonym of 'swarm').
'on the way back' shows that the letters should be reversed in order.
'teem' reversed gives 'MEET'.
(Other definitions for meet that I've seen before include "Satisfy (requirements)" , "counter" , "club" , "Happen upon" , "Fitting - encounter" .)