Is French car maintaining parking rule? (8)
I believe the answer is:
estoppel
'rule?' is the definition.
(estoppel is a kind of rule)
'is french car maintaining parking' is the wordplay.
'is french' becomes 'est' ('is' in French).
'car' becomes 'opel' (I've seen this before).
'maintaining' means one lot of letters goes inside another.
'parking' becomes 'p' (abbreviation on road signs).
'opel' going around 'p' is 'oppel'.
'est'+'oppel'='ESTOPPEL'
(Other definitions for estoppel that I've seen before include "bar" , "Conclusive admission" , "Legal principle" .)