Less long (7)
I believe the answer is:
shorter
'less long' is the definition.
('short' can be the opposite of 'long' so perhaps 'shorter' is reasonable for 'less long')
This is all the clue.
(Other definitions for shorter that I've seen before include "Lower in stature" , "More lacking in length" , "More limited in duration" , "With less length" , "Not as long" .)