Not a Mister Right, it's not! (5)
I believe the answer is:
amiss
'not a mister right it's not' is the definition.
I can't judge whether this definition defines the answer.
'not a mister right it's not' is the wordplay.
'not' suggests deleting specific letters.
'a mister' becomes 'Mr' ('Mr' can be a synonym of 'mister').
'right' becomes 'arm' (right arm).
'it's not' becomes 'miss' (I can't justify this - if you can you should believe this answer much more).
'arm' with 'mr' removed is 'a'.
'a'+'miss'='AMISS'
Can you help me to learn more?
(Other definitions for amiss that I've seen before include "Unsuitable" , "Wrongly" , "Mistakenly" , "Wrorg" , "Wrong; inaccurate" .)