Wood American enters with Alan quite regularly (6)
I believe the answer is:
walnut
'wood' is the definition.
(I know that walnut is a type of wood)
'american enters with alan quite regularly' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'enters' indicates putting letters inside.
'with' becomes 'w' (short for with).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'alan'+'quite'='alanquite'
The alternate letters of 'alanquite' are 'lnut'.
'w'+'lnut'='wlnut'
'a' put inside 'wlnut' is 'WALNUT'.
(Other definitions for walnut that I've seen before include "A hardwood" , "Decorative variety of wood" , "Wrinkled tree-fruit with a hard shell" , "Hard dark-brown wood used furniture" , "Popular wood for furniture" .)