Being incapable when about to drink alcohol is not to be tolerated (13)
I believe the answer is:
unsupportable
'not to be tolerated' is the definition.
'being incapable when about to drink alcohol' is the wordplay.
'being incapable' becomes 'unable' ('unable' can be similar in meaning to 'incapable').
'when about' indicates putting letters inside.
'to drink' becomes 'sup' (I've seen this before).
'alcohol' becomes 'port' (port is a type of wine).
'sup'+'port'='support'
'unable' enclosing 'support' is 'UNSUPPORTABLE'.
'is' is the link.
(Other definitions for unsupportable that I've seen before include "Incapable of being defended" , "Not able to be tolerated" .)