Attack persistently dismal crowd in retreat (7)
I believe the answer is:
bombard
'attack persistently' is the definition.
(I've seen this before)
'dismal crowd in retreat' is the wordplay.
'dismal' becomes 'drab' (I can't justify this - if you can you should give a lot more credence to this answer).
'crowd' becomes 'mob' (mob can mean a crowd of people).
'in retreat' says the letters should be written in reverse.
'drab'+'mob'='drabmob'
'drabmob' back-to-front is 'BOMBARD'.
Can you help me to learn more?
(Other definitions for bombard that I've seen before include "Pelt with missiles" , "Organ stop" , "Attack with missiles" , "Attack persistently" , "Batter heavily and persistently" .)