A very old firm maintains poster for fruit (7)
I believe the answer is:
avocado
'fruit' is the definition.
(avocado is a kind of fruit)
'a very old firm maintains poster' is the wordplay.
'very' becomes 'v' (abbreviation).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'firm' becomes 'co' (a firm is a company).
'maintains' indicates putting letters inside.
'poster' becomes 'ad' (short for advert).
'co' placed around 'ad' is 'cado'.
'a'+'v'+'o'+'cado'='AVOCADO'
'for' acts as a link.
(Other definitions for avocado that I've seen before include "Pulpy fruit" , "Savoury pear-shaped fruit" , "A fruit; a dull green" , "Ingredient of guacamole" , "Tropical salad ingredient" .)