Architect's design's ending with wood (4)
I believe the answer is:
nash
'architect's' is the definition.
(I've seen this before)
'design's ending with wood' is the wordplay.
'ending' says to take the final letters (the ending of the word).
'with' says to put letters next to each other.
'wood' becomes 'ash' (wood from an ash tree).
The final letter of 'design' is 'n'.
'n'+'ash'='NASH'
(Other definitions for nash that I've seen before include "John -, Eng. architect" , "Uncontrollable emotion" , "Ogden --, US humorous poet" , "Famous dandy, Beau --" , "English architect, d. 1835 - American poet, d. 1971" .)