Belay travelling over weight regulation (6)
I believe the answer is:
byelaw
'regulation' is the definition.
Both the answer and definition are singular nouns.
Perhaps you can see an association between them that I can't see?
'belay travelling over weight' is the wordplay.
'travelling' indicates an anagram.
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'weight' becomes 'w' (short for weight).
'belay' anagrammed gives 'byela'.
'byela'+'w'='BYELAW'
Can you help me to learn more?
(Other definitions for byelaw that I've seen before include "Rule made by a local authority" , "Local authority regulation" .)