Stacks concealing a pig in wood (8)
I believe the answer is:
mahogany
'wood' is the definition.
(I know that mahogany is a type of wood)
'stacks concealing a pig' is the wordplay.
'stacks' becomes 'many' (stacks can mean many of something).
'concealing' means one lot of letters goes inside another.
'pig' becomes 'hog' (synonyms).
'a'+'hog'='ahog'
'many' enclosing 'ahog' is 'MAHOGANY'.
'in' acts as a link.
(Other definitions for mahogany that I've seen before include "tree" , "Reddish-brown wood" , "Dark wood" , "Hard tropical wood used in making furniture" , "Wooden" .)