Seen in Roman square, a once leading British female missionary (6)
I believe the answer is:
xavier
'missionary' is the definition.
(I know that Xavier is a missionary)
'seen in roman square a once leading british female' is the wordplay.
'seen in' means one lot of letters goes inside another.
'roman square' becomes 'XVI' (Roman numerals for 16, a square number).
'once leading british female' becomes 'ER' (Elizabeth Regina ie Queen Elizabeth).
'xvi' placed around 'a' is 'xavi'.
'xavi'+'er'='XAVIER'
(Other definitions for xavier that I've seen before include "St Francis . . . . . ., founder Jesuit" , "St Francis ---, Apostle of the Indies" , "St Francis -, sixteenth century Spanish Jesuit missionary" , "St Francis ...... helped found the Jesuits" , "Catholic missionary" .)