Behind both sides, fencing area on one side (7)
I believe the answer is:
lateral
'one side' is the definition.
(I've seen this in another clue)
'behind both sides fencing area' is the wordplay.
'behind' becomes 'late' (as in behind schedule).
'both sides' becomes 'RL' (abbreviation for left and right).
'fencing' means one lot of letters goes inside another (to fence can mean to contain or put a limit round).
'area' becomes 'a' (maths abbreviation).
'rl' enclosing 'a' is 'ral'.
'late'+'ral'='LATERAL'
'on' is the link.
(Other definitions for lateral that I've seen before include "One type of thinking" , "to one side" , "This kind of thinking is unconventional" , "Edgeways" , "Sort of thinking" .)