Nonsense about very large perch (5)
I believe the answer is:
roost
'perch' is the definition.
(I know that perch can be written as roost)
'nonsense about very large' is the wordplay.
'nonsense' becomes 'rot' (rot can mean nonsense or rubbish).
'about' means one lot of letters goes inside another.
'very large' becomes 'os' (abbreviation for oversize).
'rot' going around 'os' is 'ROOST'.
(Other definitions for roost that I've seen before include "(Of birds) settle for rest" , "Place where birds rest" , "unwelcome to come home to?" , "Perch for a sleeping bird" , "Where birds rest at night" .)