Trailblazer, one imprisoned by sanctimonious queen (7)
I believe the answer is:
pioneer
'trailblazer' is the definition.
(thesaurus)
'one imprisoned by sanctimonious queen' is the wordplay.
'imprisoned by' indicates putting letters inside.
'sanctimonious' becomes 'pi'.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'pi'+'er'='pier'
'one' inserted into 'pier' is 'PIONEER'.
(Other definitions for pioneer that I've seen before include "Open up an area" , "First explorer" , "spacecraft" , "Trailblazer - initiate" , "first time in the field" .)