Part of forgotten thespian (5)
I believe the answer is:
tenth
'part' is the definition.
(I've seen this before)
'of forgotten thespian' is the wordplay.
'of' indicates a hidden word.
'TENTH' is hidden within 'forgotten thespian'.
(Other definitions for tenth that I've seen before include "One behind ninth" , "Tithe proportion" , "(Decimal?) fraction" , "Ordinal number" , "0.1" .)