Recalled first regiment breaking bones (5)
I believe the answer is:
tarsi
'bones' is the definition.
(bones of the foot)
'recalled first regiment breaking' is the wordplay.
'recalled' shows that the letters should be reversed in order (to recall can mean to bring something back).
'first' becomes 'ist' (ist resembles 1st).
'regiment' becomes 'RA' (Royal Artillery).
'breaking' is an insertion indicator.
'ist' enclosing 'ra' is 'israt'.
'israt' backwards is 'TARSI'.
(Other definitions for tarsi that I've seen before include "collections of bones?" , "Ankle bones" , "Parts of skeleton" , "parts of leg" , "Foot bones" .)