Understand Queen's upset these days (4)
I believe the answer is:
read
'understand' is the definition.
('read' can be a synonym of 'understand')
'queen's upset these days' is the wordplay.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'upset' shows that the letters should be reversed in order.
'these days' becomes 'ad' (as in Anno Domini, after Christ).
'er' reversed gives 're'.
're'+'ad'='READ'
(Other definitions for read that I've seen before include "Understand written music" , "Con" , "Speak" , "register" , "examined" .)