Sailors or an officer in special service (5)
I believe the answer is:
salts
'sailors' is the definition.
(salt can informally mean a sailor)
'an officer in special service' is the wordplay.
'an officer' becomes 'lt' (abbreviation for lieutenant).
'in' is an insertion indicator.
'special service' becomes 'SAS' (Special Air Service).
'lt' put into 'sas' is 'SALTS'.
'or' acts as a link.
(Other definitions for salts that I've seen before include "Six [EPSOM]'s might revive" , "Medicinal compounds; Epsom --" , "Epsom--; smelling --" , "Old sailors" , "Saline draughts" .)