Plan first part of play in theatre complex initially (6)
I believe the answer is:
tactic
'plan' is the definition.
(tactic is a kind of plan)
'first part of play in theatre complex initially' is the wordplay.
'first part of play' becomes 'Act I' (Act 1 in a play).
'in' is an insertion indicator.
'initially' suggests taking the first letters.
The first letters of 'theatre complex' is 'tc'.
'acti' put within 'tc' is 'TACTIC'.
(Other definitions for tactic that I've seen before include "A manoeuvre or planned action" , "Strategy carefully planned to achieve a specific end" , "Device" , "Military means to an end" , "An action planned to achieve a specific end" .)