Strategic opening of play in tense state (8)
I believe the answer is:
tactical
'strategic' is the definition.
(using certain tactics)
'opening of play in tense state' is the wordplay.
'opening of play' becomes 'Act I' (first act of a play).
'in' indicates putting letters inside.
'tense' becomes 't' (abbreviation used in many dictionaries).
'state' becomes 'cal' (short for California).
't'+'cal'='tcal'
'acti' placed inside 'tcal' is 'TACTICAL'.
(Other definitions for tactical that I've seen before include "Relating to manoeuvres in the presence of the enemy" , "Of carefully planned actions to achieve ends" , "Done to support skilful planning" , "according to plan" , "Strategic" .)