Old variety of fruit (6)
I believe the answer is:
orange
'fruit' is the definition.
(orange is a kind of fruit)
'old variety' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'variety' becomes 'range' (I've seen this before).
'o'+'range'='ORANGE'
'of' acts as a link.
(Other definitions for orange that I've seen before include "Type of fruit from which bergamot comes" , "Belisha beacon colour" , "juicy snack" , "Citrus fruit; colour" , "Kind of sauce" .)