Actors are remarkably deaf to it (5)
I believe the answer is:
aside
'actors are remarkably deaf to it' is the definition.
I can't judge whether this defines the answer.
'are remarkably deaf to it' is the wordplay.
'are' becomes 'a' (short for 'are', historical unit of measurement).
'remarkably deaf' becomes 'side' (I can't explain this - if you can you should give a lot more credence to this answer).
'to it' says to put letters next to each other.
'a'+'side'='ASIDE'
Can you help me to learn more?
(Other definitions for aside that I've seen before include "Aides (anag.)" , "In reserve; actor's remark to audience" , "Quiet words spoken" , "Privately, apart" , "Out of other people's hearing" .)