Nervy old hosts backing mime artist (8)
I believe the answer is:
agitated
'nervy' is the definition.
(thesaurus)
'old hosts backing mime artist' is the wordplay.
'old' becomes 'aged' ('aged' can be similar in meaning to 'old').
'hosts' indicates putting letters inside.
'backing' says the letters should be written in reverse.
'mime artist' becomes 'tati' (French mime Jacques Tati).
'tati' backwards is 'itat'.
'aged' enclosing 'itat' is 'AGITATED'.
(Other definitions for agitated that I've seen before include "Upset (by a stirrer?)" , "Jittery, hyped up" , "Worried" , "caused a stir" , "Tried to stir up public opinion" .)