Midday about time to shore up river barge (7)
I believe the answer is:
pontoon
'river barge' is the definition.
'pontoon' can be an answer for 'barge' (pontoon is a kind of barge). I am not sure about the 'river' bit.
'midday about time to shore up' is the wordplay.
'midday' becomes 'noon' (synonyms).
'about' indicates putting letters inside.
'time' becomes 't' (abbreviation).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'shore up' becomes 'po' (I can't explain this - if you can you should believe this answer much more).
'noon' enclosing 't' is 'ntoon'.
'ntoon' put after 'po' is 'PONTOON'.
Can you help me to learn more?
(Other definitions for pontoon that I've seen before include "Gambling game" , "Card game; float" , "Vingt-et-un" , "Flat-bottomed boat supporting bridge" , "Card game; floating support" .)