Old mountains producing fruit (6)
I believe the answer is:
orange
'fruit' is the definition.
(orange is a kind of fruit)
'old mountains' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'mountains' becomes 'range' (I've seen this before).
'o'+'range'='ORANGE'
'producing' is the link.
(Other definitions for orange that I've seen before include "Colour of an onager?" , "Onager (anag.)" , "Citrus fruit; colour" , "tree" , "Unrhymable word?" .)