Sailor's art form in middle-age (6)
I believe the answer is:
rating
'sailor's' is the definition.
(British term for a naval enlisted man)
'art form in middle-age' is the wordplay.
'form' is an anagram indicator.
'middle' indicates the central letters.
The middle of 'age' is 'g'.
'art' anagrammed gives 'rat'.
'rat'+'in'+'g'='RATING'
(Other definitions for rating that I've seen before include "Classification - or a non-com sailor" , "Rank - sailor" , "Salt" , "Valuation" , "Angry reprimand for non-commissioned sailor?" .)