Perhaps bay tree lacks width (4)
I believe the answer is:
roan
'perhaps bay' is the definition.
(horse with particular type of coat)
'tree lacks width' is the wordplay.
'tree' becomes 'rowan' (I've seen this before).
'lacks' is a deletion indicator.
'width' becomes 'w'.
'rowan' with 'w' taken away is 'ROAN'.
(Other definitions for roan that I've seen before include "Horse with main colour interspersed with hairs of another" , "(Horse) white-sprinkled coat" , "Horse with reddish-brown coat" , "Horse - leather" , "Nora (anag) - horse" .)