A very old company securing publicity for fruit (7)
I believe the answer is:
avocado
'for fruit' is the definition.
(avocado is a kind of fruit)
'a very old company securing publicity' is the wordplay.
'very' becomes 'v' (abbreviation).
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'company' becomes 'co' (abbreviation).
'securing' means one lot of letters goes inside another.
'publicity' becomes 'ad' (short for advert).
'co' going around 'ad' is 'cado'.
'a'+'v'+'o'+'cado'='AVOCADO'
(Other definitions for avocado that I've seen before include "Evergreen" , "Greenish-colour" , "Light green; fruit" , "Tropical salad ingredient" , "Tropical fruit with a yellow creamy flesh" .)