Driving mounted policemen around a lot (7)
I believe the answer is:
dynamic
'driving' is the definition.
('dynamic' can be similar in meaning to 'driving')
'mounted policemen around a lot' is the wordplay.
'mounted' says the letters should be written in reverse.
'policemen' becomes 'cid' (I've seen this in other clues).
'around' means one lot of letters goes inside another.
'a lot' becomes 'many' (**).
'cid' placed around 'many' is 'cimanyd'.
'cimanyd' reversed gives 'DYNAMIC'.
(Other definitions for dynamic that I've seen before include "Vigorous and go-ahead" , "Constantly changing" , "High-powered" , "Driving" , "Vigorous and effective" .)