Ultimately defines actor as one acting other than they are (4)
I believe the answer is:
sham
'one acting other than they are' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'ultimately defines actor' is the wordplay.
'ultimately' indicates one should take the final letters.
'actor' becomes 'ham' (ham is a kind of actor).
The final letter of 'defines' is 's'.
's'+'ham'='SHAM'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for sham that I've seen before include "Cheap falseness" , "Pretence, hoax" , "Pretence; bogus, false" , "Mock" , "charade" .)