Authorised measure preceding bitter explanation (9)
I believe the answer is:
rationale
'explanation' is the definition.
(rationale is a kind of explanation)
'authorised measure preceding bitter' is the wordplay.
'authorised measure' becomes 'ration' (a ration is an authorised amount of something).
'preceding' says to put letters next to each other.
'bitter' becomes 'ale' (type of beer or bitter).
'ration'+'ale'='RATIONALE'
(Other definitions for rationale that I've seen before include "Logic, reasoning" , "The logical basis of some thing or action" , "solution in theory" , "Motivation" , "Why regulate" .)