Body of water that could be the source of the Nile? (5)
I believe the answer is:
inlet
'body of water' is the definition.
(inlet is a kind of body of water)
'could be the source of the nile?' is the wordplay.
'could be' indicates anagramming the letters.
'the source of' says to take the initial letters.
The initial letter of 'the' is 't'.
't'+'nile'='tnile'
'tnile' with letters rearranged gives 'INLET'.
'that' is the link.
(Other definitions for inlet that I've seen before include "A small creek or cove" , "Entry; arm of the sea" , "bit of a lake here?" , "Opening through which liquid can flow" , "Sound" .)