Some lowlife recalled throttling constant hanger-on (5)
I believe the answer is:
leech
'hanger-on' is the definition.
(leech can mean a hanger-on or parasite)
'some lowlife recalled throttling constant' is the wordplay.
'some lowlife' becomes 'heel' (I've seen this in another clue).
'recalled' shows that the letters should be reversed in order.
'throttling' means one lot of letters goes inside another.
'constant' becomes 'c' (mathematical abbreviation).
'heel' in reverse letter order is 'leeh'.
'leeh' enclosing 'c' is 'LEECH'.
(Other definitions for leech that I've seen before include "Annelid worm, used in medicine of old" , "Worm - sponger" , "Blood-sucking creature" , "Old medicinal sucker" , "Proverbial bloodsucker" .)