We object — A&E's good in practice (5)
I believe the answer is:
usage
'practice' is the definition.
(usage is a kind of practice)
'we object ae's good in' is the wordplay.
'we object' becomes 'us' (object pronoun for 'we').
'good' becomes 'g' (abbreviation).
'in' means one lot of letters goes inside another.
'ae' going around 'g' is 'age'.
'us'+'age'='USAGE'
(Other definitions for usage that I've seen before include "Terminology - custom" , "Habit" , "Utilisation" , "Employment - practice" , "Custom - treatment" .)