Queen perform accordingly (4)
I believe the answer is:
ergo
'accordingly' is the definition.
('ergo' can be a synonym of 'accordingly')
'queen perform' is the wordplay.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'perform' becomes 'go'.
'er'+'go'='ERGO'
(Other definitions for ergo that I've seen before include "for this reason" , "Hence" , "Consequently" , "It follows" , "'Therefore, to the old Romans (4)'" .)