Mob on move did crawl across river? (5)
I believe the answer is:
swarm
'mob on move' is the definition.
(eg a swarm of insects flying around)
'did crawl across river?' is the wordplay.
'did crawl' becomes 'swam' (crawl is a type of swimming).
'across' is an insertion indicator.
'river?' becomes 'r' (abbreviation for river).
'swam' going around 'r' is 'SWARM'.
(Other definitions for swarm that I've seen before include "Throng of insects" , "Group of flies" , "Plague of insects" , "Go to form new colony" , "Large number" .)