Post first-class to reach Scottish island (6)
I believe the answer is:
staffa
'scottish island' is the definition.
(I've seen this before)
'post first-class' is the wordplay.
'post' becomes 'staff' (I've seen this before).
'first class' becomes 'A' (as in A-list, A-team etc).
'staff'+'a'='STAFFA'
'to reach' acts as a link.
(Other definitions for staffa that I've seen before include "Fingal's cave isle" , "Hebridean isle" , "small uninhabited island" , "Island west of Mull" , "Hebridean island: site of Fingal's Cave" .)