Used to be a revolutionary tool (3)
I believe the answer is:
was
'used to be' is the definition.
(eg 'he was...' means 'he used to be...')
'a revolutionary tool' is the wordplay.
'a revolutionary' says the letters should be written in reverse (letters revolve around).
'tool' becomes 'saw' (saw is a kind of tool).
'saw' reversed gives 'WAS'.
(Other definitions for was that I've seen before include "(He/she/it) existed" , "(I/he) existed" , "(He/she) existed" , "Did live" , "Lived" .)