Plan start of play set in Titanic's hull? (6)
I believe the answer is:
tactic
'plan' is the definition.
(tactic is a kind of plan)
'start of play set in titanic's hull?' is the wordplay.
'start of play' becomes 'acti' (Act I).
'set in' indicates putting letters inside.
'hull?' suggests removing the centre (the sides of a ship are part of its hull).
'titanic' with its centre taken out is 'tc'.
'acti' put within 'tc' is 'TACTIC'.
(Other definitions for tactic that I've seen before include "Skilful move or plan" , "Scheme or ploy used in battle, sport, etc, often within an overall strategy" , "Military means to an end" , "Device" , "Ruse, strategy" .)