Beginning to catch small perch (5)
I believe the answer is:
roost
'perch' is the definition.
('roost' can be a synonym of 'perch')
'beginning to catch small' is the wordplay.
'beginning' becomes 'root' ('root' can be a synonym of 'beginning'**).
'to catch' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'root' placed around 's' is 'ROOST'.
(Other definitions for roost that I've seen before include "In Orkney, the current" , "Where birds rest at night" , "Bird's night-time perch" , "Torso (anag)" , "Colony of bats; perch" .)