Miles a bit unworthy, essentially, to become ring fighter (7)
I believe the answer is:
matador
'ring fighter' is the definition.
'miles a bit unworthy essentially' is the wordplay.
'miles' becomes 'm' (common abbreviation).
'bit' becomes 'tad' (both can mean a small amount).
'essentially' says to take the centre.
The central letters of 'unworthy' are 'or'.
'm'+'a'+'tad'+'or'='MATADOR'
'to become' is the link.
(Other definitions for matador that I've seen before include "A Spanish bullfighter" , "ROM data (anag)" , "paid killer" , "fighter in the ring?" , "Beastly fighting man" .)