Nonsense written about very large perch (5)
I believe the answer is:
roost
'perch' is the definition.
('roost' can be a synonym of 'perch')
'nonsense written about very large' is the wordplay.
'nonsense' becomes 'rot' (I've seen this before).
'written about' indicates putting letters inside.
'very large' becomes 'os' (abbreviation for oversize).
'rot' placed around 'os' is 'ROOST'.
(Other definitions for roost that I've seen before include "Where bats congregate" , "Bird's shelter" , "unwelcome to come home to?" , "bed" , "Where birds rest at night" .)