Dog seen beside river boat (7)
I believe the answer is:
collier
'boat' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'dog seen beside river' is the wordplay.
I cannot really see how this works, but
'dog' could be 'collie' (collie is a kind of dog) and 'collie' is present in the answer.
'river' could be 'r' (abbreviation) and 'r' is located in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for collier that I've seen before include "Ship carrying coal" , "Worker down under" , "Pitman" , "He works in a coal mine" , "Geordie Jack, commonly" .)