Mediocre performer wanting praise, forgetting line after overacting (9)
I believe the answer is:
hamfatter
'mediocre performer' is the definition.
(hamfatter can mean a terrible actor)
'praise forgetting line after overacting' is the wordplay.
'praise' becomes 'flatter' (to flatter someone is to praise them insincerely).
'forgetting' is a deletion indicator.
'line' becomes 'l' (used when specifying particular lines from a poem).
'after' means one lot of letters go next to another.
'overacting' becomes 'ham' (as in a ham actor).
'flatter' with 'l' taken away is 'fatter'.
'fatter' after 'ham' is 'HAMFATTER'.
'wanting' acts as a link.
(Other definitions for hamfatter that I've seen before include "dramatic failure?" , "Third-rate artiste" .)