The whiskey she's short of isn't Scotch (4)
I believe the answer is:
iris
'the whiskey' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'she's short of isn't scotch' is the wordplay.
'she' becomes 'ir' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'short of' suggests taking the first letters (I've seen 'short' mean this).
The first letters of 'isnt scotch' is 'is'.
'ir'+'is'='IRIS'
Can you help me to learn more?
(Other definitions for iris that I've seen before include "One controlling pupil' s reactions" , "Flag - part of eye" , "In the middle of which pupil is opening" , "bloomer?" , "bit of mince pie" .)