When erected, integral nest boxes expand (7)
I believe the answer is:
enlarge
'expand' is the definition.
(thesaurus)
'when erected integral nest boxes' is the wordplay.
'when erected' says the letters should be written in reverse (in a down clue, letters go up).
'boxes' indicates a hidden word (to box can mean to contain).
'egralne' can be found hidden inside 'integral nest'.
'egralne' reversed gives 'ENLARGE'.
(Other definitions for enlarge that I've seen before include "Stretch" , "Make bigger, say photo" , "Make bigger in general" , "Widen" , "Make big" .)