Waterway used for splendid marriage by Conservative - a Northern man (5,5,5)
I believe the answer is:
grand union canal
'conservative a northern man' is the definition.
I can't judge whether this definition defines the answer.
'waterway used for splendid marriage' is the wordplay.
I cannot really see how this works, but
'waterway' could be 'canal' (I've seen this before) and 'canal' is present in the answer.
'splendid' could be 'grand' (both can mean impressive) and 'grand' is present in the answer.
'marriage' could be 'union' (synonyms) and 'union' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'by' is the link.
Can you help me to learn more?
(Other definitions for grand union canal that I've seen before include "Way with water" , "watercourse" .)