As a substitute it could be stained (7)
I believe the answer is:
instead
'as a substitute' is the definition.
(I've seen this before)
'it could be stained' is the wordplay.
'it could be' indicates an anagram (the same letters could be an anagram).
'stained' anagrammed gives 'INSTEAD'.
(Other definitions for instead that I've seen before include "Rather - detains (anag)" , "Stained (anag)" , "As a substitute" , "for a change" , "use this as a replacement" .)