A very old blighter, round for fruit (7)
I believe the answer is:
avocado
'fruit' is the definition.
(I know that avocado is a type of fruit)
'a very old blighter round' is the wordplay.
'very' becomes 'v' (abbreviation).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'blighter' becomes 'cad' (both can mean an unpleasant person).
'round' becomes 'o' (round can mean a round object).
'a'+'v'+'o'+'cad'+'o'='AVOCADO'
'for' is the link.
(Other definitions for avocado that I've seen before include "may be stoned" , "Tropical tree - edible green-skinned fruit" , "Light green colour" , "one's stoned?" , "Savoury pear-shaped fruit" .)