Gypsy hiding facts about killer (7)
I believe the answer is:
matador
'killer' is the definition.
(I've seen this before)
'gypsy hiding facts about' is the wordplay.
'gypsy' becomes 'rom' (a Rom is a member of the Roma people).
'hiding' means one lot of letters goes inside another.
'facts' becomes 'data' (data are bits of information).
'about' says the letters should be written backwards.
'rom' enclosing 'data' is 'rodatam'.
'rodatam' back-to-front is 'MATADOR'.
(Other definitions for matador that I've seen before include "Spaniard that fights" , "One in the ring" , "fighter in the ring?" , "who does for bulls" , "paid killer" .)