Chart river in opening at harbour (5)
I believe the answer is:
graph
'chart' is the definition.
(I know that chart can be written as graph)
'river in opening at harbour' is the wordplay.
I cannot really see how this works, but
'river' could be 'r' (abbreviation) and 'r' is present in the answer.
'opening' could be 'gap' (synonyms) and 'gap' is located in the remaining letters.
A single letter 'h' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for graph that I've seen before include "Symbolic diagram" , "A plotted chart" , "Mathematical diagram" , "Mathematical plot" , "Plotted diagram" .)