Insects paired off across top of trees (7)
I believe the answer is:
diptera
'insects' is the definition.
(I've seen this before)
'paired off across top of trees' is the wordplay.
'off' indicates anagramming the letters.
'across' means one lot of letters goes inside another.
'top of' says to take the initial letters.
The first letter of 'trees' is 't'.
'paired' is an anagram of 'dipera'.
'dipera' enclosing 't' is 'DIPTERA'.
(Other definitions for diptera that I've seen before include "Insects" , "Flighty little types" , "Flies" .)