Those heartless soldiers returned to arrest Navy SEAL (6)
I believe the answer is:
signet
'seal' is the definition.
(signet is a kind of seal)
'those heartless soldiers returned to arrest navy' is the wordplay.
'heartless' suggests removing the centre.
'soldiers' becomes 'gis' (I've seen this in other clues**).
'returned' says the letters should be written backwards.
'to arrest' is an insertion indicator.
'navy' becomes 'n' (common abbreviation eg in RN Royal Navy).
'those' with its middle removed is 'te'.
'te'+'gis'='tegis'
'tegis' in reverse letter order is 'siget'.
'siget' enclosing 'n' is 'SIGNET'.
(Other definitions for signet that I've seen before include "Ingest (anag)" , "Kind of ring with seal" , "Finger-ring with engraved design" , "Tuft of hanging threads" , "Ingest in ring" .)