Reveal revolutionary memo introduced by left (3,2)
I believe the answer is:
let on
'reveal' is the definition.
('let on' can be a synonym of 'reveal')
'revolutionary memo introduced by left' is the wordplay.
'revolutionary' is a reversal indicator.
'memo' becomes 'note' (I've seen this before).
'introduced by' means one lot of letters go next to another.
'left' becomes 'l' (common abbreviation).
'note' reversed gives 'eton'.
'eton' after 'l' is 'LET ON'.
(Other definitions for let on that I've seen before include "Disclose awareness" , "Divulge info; pretend" , "Pretend, feign" , "Allow to board, or pretend" , "Reveal, divulge" .)