Posh fur fit for purpose (6)
I believe the answer is:
usable
'fit for purpose' is the definition.
(I've seen this before)
'posh fur' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'fur' becomes 'sable' (sable is a kind of fur).
'u'+'sable'='USABLE'
(Other definitions for usable that I've seen before include "Functional, handy" , "at one's disposal" , "open to exploitation" , "Practical" , "Fit for purpose" .)