Tense opening of play about plan of action? (6)
I believe the answer is:
tactic
'plan of action?' is the definition.
(tactic is a kind of plan of action)
'tense opening of play about' is the wordplay.
'tense' becomes 't' (abbreviation used in many dictionaries).
'opening of play' becomes 'Act I' (first act of a play).
'about' becomes 'c' (abbreviation for circa).
't'+'acti'+'c'='TACTIC'
(Other definitions for tactic that I've seen before include "element of strategy" , "A TT C-i-C (anag.)" , "Planned manoeuvre or skilful move" , "A skilful manoeuvre or procedure" , "Stratagem, procedure" .)