Bay tree lacking in width (4)
I believe the answer is:
roan
'bay' is the definition.
(I've seen this before)
'tree lacking in width' is the wordplay.
'tree' becomes 'rowan tree' (rowan is a kind of tree).
'lacking in' indicates named letters should be taken away.
'width' becomes 'w' (abbreviation in geometry).
'rowan' with 'w' removed is 'ROAN'.
(Other definitions for roan that I've seen before include "Horse with reddish-brown coat" , "Of horse, with coat sprinkled with white or grey hairs" , "(Horse) white-sprinkled coat" , "Bay horse with white/grey hairs in its coat" , "Horse with coat speckled with white or grey" .)