Small tree used to make window frame (4)
I believe the answer is:
sash
'window frame' is the definition.
(thesaurus)
'small tree' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'tree' becomes 'ash' (ash is a kind of tree**).
's'+'ash'='SASH'
'used to make' is the link.
(Other definitions for sash that I've seen before include "Decorative belt" , "Band worn over shoulder or round waist" , "Baldrick" , "Band of cloth" , "Long strip of material - window frame" .)