Purpose of first-class motorway (3)
I believe the answer is:
aim
'purpose' is the definition.
(synonyms)
'first-class motorway' is the wordplay.
'first class' becomes 'ai' (A1 - top quality).
'motorway' becomes 'M' (as in motorway names - Ma, M4 etc).
'ai'+'m'='AIM'
'of' is the link.
(Other definitions for aim that I've seen before include "End" , "intent" , "Set sights on, target or goal" , "Try to achieve" , "Focus on target" .)