A promontory in the mouth of the river? (6,2,4)

'river?' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'a promontory in the mouth of' is the wordplay.
I cannot really see how this works, but
'a' could be 'an' and 'an' is found within the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
'of' could be 'o' and 'o' is present in the answer.
This may be the basis of the clue (or it may be nonsense).

'the' is the link.

Can you help me to learn more?