On top of everything as a rule (7)
I believe the answer is:
overall
'as a rule' is the definition.
(thesaurus)
'on top of everything' is the wordplay.
'on top of' becomes 'over' (I've seen this in another clue).
'everything' becomes 'all' (all things).
'over'+'all'='OVERALL'
(Other definitions for overall that I've seen before include "protector" , "Comprehensive - protective garment" , "Inclusive" , "generally speaking" , "Including everything" .)