Ex-PM brewing lager with last vestiges of her late barley (4,4)
I believe the answer is:
earl grey
'ex-pm brewing' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'lager with last vestiges of her late barley' is the wordplay.
I cannot really understand how this works, but
an anagram of 'lager' is 'earlg' which is present in the answer.
'her' could be 'rey' and 'rey' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for earl grey that I've seen before include "Red cell deficiency" , "leaves much to be desired?" , "Kind of China tea" , "brew" , "leaves a lot to be desired?" .)