A mouthful of water (5)
I believe the answer is:
delta
'water' is the definition.
The answer and definition can be both natural objects as well as being singular nouns.
Maybe there's an association between them I don't understand?
'a mouthful' is the wordplay.
I cannot quite understand how this works, but
'a' is present in the answer.
The remaining letters 'delt' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'of' acts as a link.
Can you help me to learn more?
(Other definitions for delta that I've seen before include "Spreading river mouth" , "Multiple estuary" , "River mouth in the Greek alphabet" , "Fourth letter of the ancient Greek alphabet: river feature" , "Greek letter in the shape of a triangle" .)