Dash just across line (5)
I believe the answer is:
flair
'dash' is the definition.
('flair' can be a synonym of 'dash')
'just across line' is the wordplay.
'just' becomes 'fair' ('fair' is associated in meaning with 'just').
'across' is an insertion indicator.
'line' becomes 'l'.
'fair' enclosing 'l' is 'FLAIR'.
(Other definitions for flair that I've seen before include "Elan; panache" , "Style, panache" , "Natural ability/talent" , "Creative brilliance" , "skill" .)