Perch is small in origin (5)
I believe the answer is:
roost
'perch' is the definition.
('roost' can be a synonym of 'perch')
'small in origin' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'in' means one lot of letters goes inside another.
'origin' becomes 'root' ('root' can be a synonym of 'origin').
's' inserted into 'root' is 'ROOST'.
'is' is the link.
(Other definitions for roost that I've seen before include "Bird's settling place" , "bed" , "Night spot for birds/bats" , "Where birds rest at night" , "Thin paper" .)