One bargain appears the best possible (5)
I believe the answer is:
ideal
'best possible' is the definition.
(I've seen this before)
'one bargain appears' is the wordplay.
'one' becomes 'i' (Roman numeral).
'bargain' becomes 'deal' (synonyms).
'appears' is a charade indicator (letters next to each other) (some letters appear after others).
'i'+'deal'='IDEAL'
'the' acts as a link.
(Other definitions for ideal that I've seen before include "principle to be aimed at" , "existing only in imagination" , "Pattern" , "Best possible" , "superb specimen" .)