Farm, old, nevertheless is about to diversify (6,3)
I believe the answer is:
branch out
'diversify' is the definition.
('branch out' can be a synonym of 'diversify')
'farm old nevertheless is about' is the wordplay.
'farm' becomes 'ranch' (ranch is a kind of farm**).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'nevertheless' becomes 'but' (I've seen this before).
'is about' means one lot of letters goes inside another.
'ranch'+'o'='rancho'
'rancho' put inside 'but' is 'BRANCH OUT'.
'to' acts as a link.
(Other definitions for branch out that I've seen before include "Expand" , "Take up new things" , "Extend activities in new direction" , "diversify" .)