Reviewed a great deal. (4)
I believe the answer is:
stop
'deal' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Maybe you can see a link between them that I can't see?
'reviewed a great' is the wordplay.
'reviewed' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'a great' becomes 'top'.
's'+'top'='STOP'
Can you help me to learn more?
(Other definitions for stop that I've seen before include "End or pause" , "port of call" , "Hold on" , "Red light signal" , "Cease, desist, whoa" .)