One in duet — that's ridiculous! (6)
I believe the answer is:
united
'one' is the definition.
(similar in meaning)
'in duet that's ridiculous' is the wordplay.
'that's ridiculous' indicates anagramming the letters.
'in'+'duet'='induet'
'induet' is an anagram of 'UNITED'.
(Other definitions for united that I've seen before include "Manchester _____, current Premiership champions with..." , "Of like mind" , "Joined into a single entity" , "Joined together as one" , "See 18" .)