Old performer partially unconscious, in a trance (7)
I believe the answer is:
'old performer' is the definition.
'partially unconscious in a trance' is the wordplay.
'partially' indicates a hidden word.
'SINATRA' can be found hidden inside 'unconscious in a trance'.
(Other definitions for sinatra that I've seen before include "Francis Albert, mourned singer", "Frank . . . . . . ., late great popular singer", "Francis Albert, late lamented singer", "Entertainer", "'Ol' Blue Eyes'".)