Cromwell’s cavalry — mounted men breaking guts (9)
I believe the answer is:
ironsides
'cromwell's cavalry' is the definition.
(I've seen this before)
'mounted men breaking guts' is the wordplay.
'mounted' shows that the letters should be reversed in order (in a down clue, letters mounted up).
'men' becomes 'OR' (military abbreviation for Other Ranks).
'breaking' is an insertion indicator.
'guts' becomes 'insides' (both can mean innards).
'or' in reverse letter order is 'ro'.
'ro' inserted within 'insides' is 'IRONSIDES'.
(Other definitions for ironsides that I've seen before include "Several Edmunds" , "Oliver Cromwell" , "Cromwell's cavalry" , "one showing great fortitude" , "Cromwell's army" .)