Side's recent rule regularly ignored outside area (7)
I believe the answer is:
lateral
'side's' is the definition.
('lateral' can be a synonym of 'side')
'recent rule regularly ignored outside area' is the wordplay.
'recent' becomes 'late' ('late' can be similar in meaning to 'recent').
'regularly ignored' means one should take alternating letters.
'outside' indicates putting letters inside.
'area' becomes 'a' (maths abbreviation).
The alternate letters of 'rule' are 'rl'.
'rl' going around 'a' is 'ral'.
'late'+'ral'='LATERAL'
(Other definitions for lateral that I've seen before include "One type of thinking" , "Of, at or towards the sides (7)" , "Edgeways" , "Sort of thinking" , "Side branch" .)