Shy person left in river (4)
I believe the answer is:
clam
'shy person' is the definition.
(I've seen this before)
'left in river' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' indicates putting letters inside.
'river' becomes 'cam' (river in Cambridge, England).
'l' inserted into 'cam' is 'CLAM'.
(Other definitions for clam that I've seen before include "One is as uncommunicative as a mollusc" , "Tightly closed shellfish" , "shy person" , "Quahog" , "Edible shellfish" .)