Many a creature's seen in a wood (8)
I believe the answer is:
mahogany
'a wood' is the definition.
(mahogany is a kind of wood)
'many a creature's seen in' is the wordplay.
'creature' becomes 'hog' (hog is a kind of creature**).
'seen in' is an insertion indicator.
'a'+'hog'='ahog'
'many' going around 'ahog' is 'MAHOGANY'.
(Other definitions for mahogany that I've seen before include "Hard tropical wood used in making furniture" , "Old Cornish drink" , "Dark wood" , "Log" , "Type of wood" .)