Hanger-on, a slippery creature, rejected companion (5)
I believe the answer is:
leech
'hanger-on' is the definition.
(I've seen this before)
'a slippery creature rejected companion' is the wordplay.
'a slippery creature' becomes 'eel' (I've seen this before).
'rejected' says the letters should be written in reverse.
'companion' becomes 'ch' (I've seen this in other clues).
'eel' reversed gives 'lee'.
'lee'+'ch'='LEECH'
(Other definitions for leech that I've seen before include "Proverbial bloodsucker" , "Old medicinal sucker" , "Freeloader" , "Clinging type" , "One who sponges on others" .)