Despicable chap left river (5)
I believe the answer is:
louse
'despicable chap' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'left river' is the wordplay.
'left' becomes 'l' (common abbreviation).
'river' becomes 'ouse' (Yorkshire river).
'l'+'ouse'='LOUSE'
Can you help me to learn more?
(Other definitions for louse that I've seen before include "Wingless parasite" , "Insect -- nasty character, undeserving of respect" , "Parasite; nasty person" , "Mean person" , "Contemptible type" .)