Actor about to end the affair, nominally (5)
I believe the answer is:
hiram
'affair nominally' is the definition.
I can't judge whether this defines the answer.
'actor about to end' is the wordplay.
'actor' becomes 'ham' (ham actor is a kind of actor).
'about' indicates putting letters inside.
'to end' becomes 'ir' (I can't justify this - if you can you should believe this answer much more).
'ham' enclosing 'ir' is 'HIRAM'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for hiram that I've seen before include "I gave Solomon materials for Temple" , "He supplied temple builders" .)