Vouchsafed information about Roman square (5)
'vouchsafed' is the definition.
'information about roman square' is the wordplay.
'information' becomes 'gen' (gen can informally mean information).
'about' means one lot of letters goes inside another.
'roman square' becomes 'IV' (Roman numerals for four, a square number).
'gen' placed around 'iv' is 'GIVEN'.
(Other definitions for given that I've seen before include "Awarded", "assumed fact", "Taken as read", "Provided", "Handed over; an 18 dn".)