Trap to restrict boat at sea — a form of traffic control (8)
I believe the answer is:
'trap' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'to restrict boat at sea a form of traffic control' is the wordplay.
'to restrict' means one lot of letters goes inside another (inserted letters are restricted or contained).
'at sea' is an anagram indicator.
'a form of traffic control' becomes 'cage' (I can't explain this - if you can you should give a lot more credence to this answer).
'boat' anagrammed gives 'bota'.
'bota' going inside 'cage' is 'CABOTAGE'.
Can you help me to learn more?
(Another definition for cabotage that I've seen is " navigation in coastal waters".)