Reason to sing about beer? I must be drunk (9)
I believe the answer is:
rationale
'reason' is the definition.
(I've seen this before)
'sing about beer? i must be drunk' is the wordplay.
I cannot quite see how this works, but
'sing' could be 'rat' (I've seen this in another clue) and 'rat' is located in the answer.
'about' could be 'on' (as in a book 'on' a certain topic) and 'on' is found in the answer.
'beer?' could be 'ale' (type of beer) and 'ale' is found in the answer.
'i' is present in the answer.
No letters remain.
This explanation may well be incorrect...
'to' is the link.
Can you help me to learn more?
(Other definitions for rationale that I've seen before include "Logic, reasoning" , "Motivation" , "The logical basis of some thing or action" , "grounds?" , "Underlying principle" .)