Uncommunicative person left in river (4)
I believe the answer is:
clam
'uncommunicative person' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'left in river' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' means one lot of letters goes inside another.
'river' becomes 'cam' (river in Cambridge, England).
'l' going within 'cam' is 'CLAM'.
Can you help me to learn more?
(Other definitions for clam that I've seen before include "A silent type" , "Mollusc, one proverbially reticent" , "What could grip" , "Mollusc with firmly-closed shell" , "shy person" .)