A lot of insects took to the water to cross river (5)
I believe the answer is:
swarm
'a lot of insects' is the definition.
(eg a swarm of bees)
'took to the water to cross river' is the wordplay.
'took to the water' becomes 'swam' (past tense of 'swim').
'to cross' indicates putting letters inside.
'river' becomes 'r'.
'swam' enclosing 'r' is 'SWARM'.
(Other definitions for swarm that I've seen before include "Mass of flying insects" , "Climb; cluster of bees" , "Moving crowd, of bees say" , "Mob" , "Large group of bees" .)