But lots of other insects do this too (3)
I believe the answer is:
fly
'insects do this too' is the definition.
The definition suggests a singular noun which matches the answer. I also know that 'insects' relates to this answer.
I don't understand how the remainder of the clue works.
(Other definitions for fly that I've seen before include "Soar; hurry" , "One invited into parlour" , "needs to get away" , "Flee" , "An insect; run away" .)