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' says the answer is hidden in the clue.
'TENTH' can be found hidden inside 'forgotten thespian'.
(Other definitions for tenth that I've seen before include "One behind ninth" , "Tithe proportion" , "(Decimal?) fraction" , "Ordinal number" , "0.1" .)