Little creatures to get rid of, crossing river (5)
I believe the answer is:
krill
'little creatures' is the definition.
(tiny crustaceans)
'get rid of crossing river' is the wordplay.
'get rid of' becomes 'kill' (to get rid of can mean to kill).
'crossing' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation for river).
'kill' enclosing 'r' is 'KRILL'.
'to' acts as a link.
(Other definitions for krill that I've seen before include "Tiny crustaceans" , "Small aquatic creatures" , "Whales' staple diet" , "Whale food" , "Crustaceans, diet of whales" .)