Next ace left on side (7)
I believe the answer is:
lateral
'side' is the definition.
('lateral' can be a synonym of 'side')
'next ace left on' is the wordplay.
'next' becomes 'later' (I've seen this in other clues).
'ace' becomes 'a'.
'left' becomes 'l' (common abbreviation).
'on' means one lot of letters go next to another.
'later'+'a'+'l'='LATERAL'
(Other definitions for lateral that I've seen before include "Relating to the side" , "Possibly right" , "Towards the side" , "Way of thinking" , "This kind of thinking is unconventional" .)