A small coop by a tree (5)
I believe the answer is:
aspen
'tree' is the definition.
(aspen is a kind of tree)
'a small coop by a' is the wordplay.
'a small' becomes 's' (abbreviation - e.g. clothes size).
'coop' becomes 'pen' (I've seen this before).
'by' means one lot of letters go next to another.
's'+'pen'='spen'
'spen' after 'a' is 'ASPEN'.
(Other definitions for aspen that I've seen before include "city in Colorado" , "A poplar with shivering leaves" , "somewhere in Colorado" , "The tree that shivers in the slightest wind" , "that's wooden" .)