Papa gets nappy the wrong way round, with discharge already settled (7)
I believe the answer is:
prepaid
'with discharge already settled' is the definition.
(with a charge or payment settled in advance)
'papa gets nappy the wrong way round' is the wordplay.
'papa' becomes 'P' (phonetic alphabet: alpha, bravo, charlie etc.).
'gets' says to put letters next to each other.
'nappy' becomes 'diaper' (synonyms).
'the wrong way round' is a reversal indicator.
'diaper' reversed gives 'repaid'.
'p'+'repaid'='PREPAID'
(Other definitions for prepaid that I've seen before include "settled in advance" , "Settled early" .)