Nonsense! Put in a big perch! (5)
I believe the answer is:
roost
'perch' is the definition.
(synonyms)
'nonsense put in a big' is the wordplay.
'nonsense' becomes 'rot' (I've seen this in another clue).
'put in' is an insertion indicator (instruction to solver).
'a big' becomes 'os'.
'rot' enclosing 'os' is 'ROOST'.
(Other definitions for roost that I've seen before include "Place on which bird rests" , "It's sometimes ruled" , "Where birds rest at night" , "The cock rules it" , "Rest on a perch" .)