Am working until now, having had nothing but a sherry (11)
I believe the answer is:
amontillado
'sherry' is the definition.
(amontillado is a kind of sherry)
'am working until now' is the wordplay.
'working' becomes 'on' (on can mean working or activated).
'until' becomes 'till' (I've seen this in other clues).
'now' becomes 'ado' (I can't explain this - if you can you should give a lot more credence to this answer).
'am'+'on'+'till'+'ado'='AMONTILLADO'
'having had nothing but a' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for amontillado that I've seen before include "Kind of sherry" , "booze" , "Type of sherry" , "Here's sherry" , "Drink" .)