Discover an act is performed including Queen (9)
I believe the answer is:
ascertain
'discover' is the definition.
(ascertaining is a kind of discovering)
'an act is performed including queen' is the wordplay.
'performed' is an anagram indicator.
'including' is an insertion indicator.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'an'+'act'+'is'='anactis'
'anactis' is an anagram of 'asctain'.
'asctain' placed around 'er' is 'ASCERTAIN'.
(Other definitions for ascertain that I've seen before include "Cartesian (anag)" , "Establish, find out" , "Prove" , "Discover" , "Establish after an investigation or study" .)