Genius just crossing line (5)
I believe the answer is:
flair
'genius' is the definition.
('flair' can be a synonym of 'genius')
'just crossing line' is the wordplay.
'just' becomes 'fair' ('fair' is associated in meaning with 'just').
'crossing' is an insertion indicator.
'line' becomes 'l'.
'fair' going around 'l' is 'FLAIR'.
(Other definitions for flair that I've seen before include "Style, panache" , "Natural ability/talent" , "Natural talent or panache" , "Style, talent" , "Stylishness; talent" .)