Introduction of rule queried, dubiously mandatory (8)
I believe the answer is:
required
'mandatory' is the definition.
('required' can be a synonym of 'mandatory')
'introduction of rule queried dubiously' is the wordplay.
'introduction of' suggests taking the first letters.
'dubiously' indicates an anagram.
The first letter of 'rule' is 'r'.
'queried' is an anagram of 'equired'.
'r'+'equired'='REQUIRED'
(Other definitions for required that I've seen before include "Needed" , "Not optional" , "Essential, compulsory" .)