The purposes of first-class writing (4)
I believe the answer is:
aims
'the purposes' is the definition.
('aim' can be a synonym of 'purpose')
'first-class writing' is the wordplay.
'first class' becomes 'ai' (A1 - top quality).
'writing' becomes 'ms' (abbreviation for manuscript).
'ai'+'ms'='AIMS'
'of' is the link.
(Other definitions for aims that I've seen before include "Purposes or intentions" , "Points a weapon" , "objects" , "Objectives; points" , "Targets and ambitions" .)