Association of actors depart during emergency externally (6)
I believe the answer is:
equity
'association of actors' is the definition.
(trade union for the UK entertainment industry)
'depart during emergency externally' is the wordplay.
'depart' becomes 'quit' (quitting is a kind of departing).
'during' is an insertion indicator.
'externally' suggests removing the centre (only the external letters are used).
'emergency' with its middle taken out is 'ey'.
'quit' put into 'ey' is 'EQUITY'.
(Other definitions for equity that I've seen before include "Value of a property less debt" , "Fairness; shareholding" , "Group of performers" , "actors' group" , "Fair play - actors' union" .)