Lake and river insect (5)
I believe the answer is:
louse
'insect' is the definition.
(louse is a kind of insect)
'lake and river' is the wordplay.
'lake' becomes 'l'.
'and' is a charade indicator (letters next to each other).
'river' becomes 'ouse' (Yorkshire river**).
'l'+'ouse'='LOUSE'
(Other definitions for louse that I've seen before include "Wingless parasitic insect" , "Nit" , "scum" , "25 [PARASITE]" , "Wingless bloodsucking insect" .)