Heavy blow toppled English prisoner (4)
I believe the answer is:
gale
'heavy blow' is the definition.
(I've seen this in another clue)
'toppled english prisoner' is the wordplay.
'toppled' shows that the letters should be reversed in order (in a down clue, letters go up).
'english' becomes 'E' (abbreviation).
'prisoner' becomes 'lag' (slang term).
'e'+'lag'='elag'
'elag' reversed gives 'GALE'.
(Other definitions for gale that I've seen before include "A storm of wind and rain" , "High wind" , "A wild storm" , "Fierce wind" , "Squall" .)