Part of tree containing small branch for resting birds (5)
I believe the answer is:
roost
'branch for resting birds' is the definition.
I can't tell whether this definition defines the answer.
'part of tree containing small' is the wordplay.
'part of tree' becomes 'root' (trees have roots).
'containing' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'root' enclosing 's' is 'ROOST'.
Can you help me to learn more?
(Other definitions for roost that I've seen before include "Bird's shelter" , "The cock rules it" , "Temporary resting-place" , "Bird's settling place" , "Place on which bird rests" .)