One minute before someone in theatre, perhaps, is restless (9)
I believe the answer is:
impatient
'is restless' is the definition.
(similar in meaning)
'one minute before someone in theatre' is the wordplay.
'one' becomes 'i' (Roman numeral).
'minute' becomes 'm' (abbreviation).
'before' is a charade indicator (letters next to each other).
'someone in theatre' becomes 'patient' (surgical patient in an operating theatre).
'i'+'m'+'patient'='IMPATIENT'
'perhaps' is the link.
(Other definitions for impatient that I've seen before include "Keen" , "Restless or short of temper" , "Fidgety" , "Restlessly eager" , "showing anxiety" .)