Jack's death curtailed by Leo, perhaps (6)
I believe the answer is:
ensign
'jack's' is the definition.
(thesaurus)
'death curtailed by leo perhaps' is the wordplay.
'death' becomes 'end' (synonyms).
'curtailed' means to remove the last letter.
'by' says to put letters next to each other.
'leo perhaps' becomes 'sign' (leo is a kind of sign).
'end' with its last letter removed is 'en'.
'en'+'sign'='ENSIGN'
(Other definitions for ensign that I've seen before include "Low-ranking naval officer" , "Small flag or junior naval officer" , "Junior US naval officer" , "Flag - junior American naval officer" , "Lowly naval officer" .)