Grub catching small perch (5)
I believe the answer is:
roost
'perch' is the definition.
('roost' can be a synonym of 'perch')
'grub catching small' is the wordplay.
'grub' becomes 'root' (I've seen this before).
'catching' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'root' enclosing 's' is 'ROOST'.
(Other definitions for roost that I've seen before include "Perch for a sleeping bird" , "somewhere to sleep" , "Perch where bird sleeps" , "unwelcome to come home to?" , "Birds' resting place" .)