Curious, like returning top-class Japanese fighter once (7)
I believe the answer is:
samurai
'japanese fighter once' is the definition.
'curious like returning top-class' is the wordplay.
'curious' becomes 'rum' ('rum' can be a synonym of 'curious').
'like' becomes 'as'.
'returning' is a reversal indicator.
'top-class' becomes 'ai' (resembles A1, informal term for top quality).
'rum'+'as'='rumas'
'rumas' in reverse letter order is 'samur'.
'samur'+'ai'='SAMURAI'
(Other definitions for samurai that I've seen before include "Japanese fighter" , "military type" , "Feudal Japanese warrior" , "Japanese military retainer" , "Member of the military caste in feudal Japan" .)