One rejected song penned by Papa Haydn initially (6)
I believe the answer is:
pariah
'one' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'rejected song penned by papa haydn initially' is the wordplay.
'rejected' shows that the letters should be reversed in order.
'song' becomes 'air' (air can mean a piece of music).
'penned by' is an insertion indicator (inserted letters are penned in).
'papa' becomes 'pa' (synonyms).
'initially' says to take the initial letters.
The first letter of 'haydn' is 'h'.
'air' reversed gives 'ria'.
'pa'+'h'='pah'
'ria' placed into 'pah' is 'PARIAH'.
Can you help me to learn more?
(Other definitions for pariah that I've seen before include "Small box for valuables" , "Unacceptable person" , "person shunned" , "Person despised and rejected by society" , "Outcast (state)" .)