Hanger-on has left and I'm favourite (6)
I believe the answer is:
limpet
'hanger-on' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'left and i'm favourite' is the wordplay.
'left' becomes 'l' (common abbreviation).
'and' is a charade indicator (letters next to each other).
'favourite' becomes 'pet' (synonyms).
'l'+'im'+'pet'='LIMPET'
'has' is the link.
Can you help me to learn more?
(Other definitions for limpet that I've seen before include "A shellfish that sticks to rocks or ships" , "Mollusc that clings to rocks" , "Shellfish; type of mine" , "Clinging gastropod" , "clingy type" .)