Best knitted top, one buttoned up (7)
I believe the answer is:
optimum
'best' is the definition.
(optimum means the best possible)
'knitted top one buttoned up' is the wordplay.
'knitted' indicates an anagram (knit can mean to screw up or crease).
'one' becomes 'i' (Roman numeral).
'buttoned up' becomes 'mum' (I've seen this in another clue).
'top' anagrammed gives 'opt'.
'opt'+'i'+'mum'='OPTIMUM'
(Other definitions for optimum that I've seen before include "Most appropriate" , "Most favourable amount" , "Best (result)" , "top condition" , "Most favourable (conditions)" .)