Small timber frame (4)
I believe the answer is:
sash
'frame' is the definition.
(sash is a kind of frame)
'small timber' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'timber' becomes 'ash' (I've seen this before).
's'+'ash'='SASH'
(Other definitions for sash that I've seen before include "(Uniform) strip; window-frame" , "Part of a window - decoration" , "Decorative band" , "seen hanging around Miss World?" , "Window frame; silk band" .)