Actor's nameless brother (6)
I believe the answer is:
jacobi
'actor's' is the definition.
(I've seen this before)
'nameless brother' is the wordplay.
I cannot quite understand how this works, but
'brother' could be 'jacob' (I have seen 'Brother of Esau ' mean 'jacob' so perhaps 'brother' could also mean 'jacob') and 'jacob' is found in the answer.
A single letter 'i' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for jacobi that I've seen before include "Derek --, English actor" , "Sir Derek, British actor" .)