Number act in haste, upset about drug (11)
I believe the answer is:
anaesthetic
'number act' is the definition.
'anaesthetic' can be an answer for 'number' (I've seen this before). I'm not sure about the 'act' bit.
'haste upset about drug' is the wordplay.
'upset' indicates an anagram.
'about' is an insertion indicator.
'drug' becomes 'anetic' (I can't explain this - if you can you should give a lot more credence to this answer).
'haste' with letters rearranged gives 'aesth'.
'aesth' placed into 'anetic' is 'ANAESTHETIC'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for anaesthetic that I've seen before include "Drug that causes temporary loss of feeling" , "used in theatre" , "A number of theatres" , "Knockout drug" , "Number of bodies - can hesitate (anag)" .)