Cavalry is pressing round flank (9)
I believe the answer is:
ironsides
'cavalry' is the definition.
(term for Cromwell's cavalry in English Civil War)
'is pressing round flank' is the wordplay.
'is pressing' becomes 'irons' (to iron is to press creases out of clothes).
'round' means one lot of letters goes inside another.
'flank' becomes 'side' (I've seen this before).
'irons' enclosing 'side' is 'IRONSIDES'.
(Other definitions for ironsides that I've seen before include "Several Edmunds" , "Cromwell's army" , "one showing great fortitude" , "Those having great stamina" , "Cromwell's cavalry" .)