A very old poster in firm's green (7)
I believe the answer is:
avocado
'green' is the definition.
(shade of green)
'a very old poster in firm's' is the wordplay.
'very' becomes 'v' (abbreviation).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'poster' becomes 'ad' (short for advert).
'in' is an insertion indicator.
'firm' becomes 'co' (a firm is a company).
'ad' inserted into 'co' is 'cado'.
'a'+'v'+'o'+'cado'='AVOCADO'
(Other definitions for avocado that I've seen before include "Blackish- or yellowish-green colour" , "Tropical fruit with a yellow creamy flesh" , "Tropical salad ingredient" , "Dull greenish colour" , "may be stoned" .)