What is neat on a river — a new boat (8)
I believe the answer is:
trimaran
'boat' is the definition.
(thesaurus)
'what is neat on a' is the wordplay.
'what is' becomes 'ar' (I can't explain this - if you can you should give a lot more credence to this answer).
'neat' becomes 'trim' ('trim' can be a synonym of 'neat').
'on' is a charade indicator (letters next to each other).
'a' becomes 'an'.
'ar' after 'trim' is 'trimar'.
'trimar'+'an'='TRIMARAN'
'river a new' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
(Other definitions for trimaran that I've seen before include "Three-hulled boat" , "type of boat" , "Three-hulled craft" , "Sailing vessel" , "Three-hulled vessel" .)