One superior form of tin (4)
I believe the answer is:
unit
'one' is the definition.
(synonyms)
'superior form of tin' is the wordplay.
'superior' becomes 'u' (U can mean upper-class, as in U and non-U speech).
'form of' is an anagram indicator (the letters need to be in another 'form').
'tin' with letters rearranged gives 'nit'.
'u'+'nit'='UNIT'
(Other definitions for unit that I've seen before include "Kitchen element?" , "squad" , "Small military detachment" , "Kitchen fitting" , "Group that's part of a larger organisation" .)