Sprite's pretty unknown (5)
I believe the answer is:
fairy
'sprite's' is the definition.
(synonyms)
'pretty unknown' is the wordplay.
'pretty' becomes 'fair' (both can mean beautiful or attractive).
'unknown' becomes 'y' (algebra).
'fair'+'y'='FAIRY'
(Other definitions for fairy that I've seen before include "eg Puck" , "Cobweb, perhaps" , "Magical being" , "Sprite with a toadstool for a home" , "sort of story?" .)