Aquatic insect (5,7)
I believe the answer is:
water boatman
'aquatic insect' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for water boatman that I've seen before include "Carnivorous aquatic bug" , "Aquatic insect" , "Floating insect" , "Pond-skimming insect" .)