Quarrel about rule with an ascetic (7)
I believe the answer is:
spartan
'ascetic' is the definition.
('spartan' can be a synonym of 'ascetic')
'quarrel about rule with an' is the wordplay.
'quarrel' becomes 'spat' (spatting is a kind of quarrelling**).
'about' indicates putting letters inside.
'rule' becomes 'r' (legal abbreviation).
'with' says to put letters next to each other.
'spat' going around 'r' is 'spart'.
'spart'+'an'='SPARTAN'
(Other definitions for spartan that I've seen before include "Indifferent to luxury" , "Not comfortable" , "Plain" , "Indifferent to comfort" , "Simple and harsh, without luxuries" .)