Queen, meeting communist, was mistaken (5)
I believe the answer is:
erred
'was mistaken' is the definition.
(I've seen this before)
'queen meeting communist' is the wordplay.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'meeting' is a charade indicator (letters next to each other).
'communist' becomes 'red' (red is a kind of communist).
'er'+'red'='ERRED'
(Other definitions for erred that I've seen before include "Made a mistake, was wrong" , "Went wrong" , "Gone astray!" , "Made mistakes" , "was inaccurate" .)