Pretty, magical creature captivates student (6)
I believe the answer is:
fairly
'pretty' is the definition.
(I know that pretty can be written as fairly)
'magical creature captivates student' is the wordplay.
'magical creature' becomes 'fairy'.
'captivates' is an insertion indicator.
'student' becomes 'l' (as in L-plates for learner drivers).
'fairy' placed around 'l' is 'FAIRLY'.
(Other definitions for fairly that I've seen before include "Evenhandedly" , "Equitably; passably" , "pretty" , "To a moderate degree" , "In an evenhanded manner - somewhat" .)