Reason address fails to start? One may be drunk (9)
I believe the answer is:
rationale
'reason' is the definition.
(thesaurus)
'address fails to start? one may be drunk' is the wordplay.
'address' becomes 'oration' (oration is a kind of address).
'fails to start?' means to remove the first letter (I've seen 'fails to start' mean this).
'one may be drunk' becomes 'ale'.
'oration' with its initial letter removed is 'ration'.
'ration'+'ale'='RATIONALE'
(Other definitions for rationale that I've seen before include "Reasoned exposition" , "Logic, reasoning" , "explanation" , "justification" , "Logical basis (for a course of action)" .)