Book set in university town in Cheshire is fit for purpose (6)
I believe the answer is:
usable
'fit for purpose' is the definition.
(I've seen this before)
'book set in university town in cheshire' is the wordplay.
'book' becomes 'b'.
'set in' means one lot of letters goes inside another.
'university' becomes 'u'.
'town in cheshire' becomes 'sale' (town historically part of Cheshire).
'u'+'sale'='usale'
'b' going within 'usale' is 'USABLE'.
'is' is the link.
(Other definitions for usable that I've seen before include "Practical" , "Fit for service" , "In working order" , "Like something functional" , "Affording some purpose" .)