Across river, a sort of bridge not available quickly (6)
I believe the answer is:
pronto
'quickly' is the definition.
(I've seen this before)
'across river a sort of bridge not' is the wordplay.
'across' indicates putting letters inside.
'river' becomes 'r' (abbreviation for river).
'a sort of bridge' becomes 'pont' (I am not sure about this - if you are sure you should believe this answer much more).
'not' becomes 'o' (I've seen this in other clues).
'r' inserted inside 'pont' is 'pront'.
'pront'+'o'='PRONTO'
'available' is the link.
This may not be correct. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for pronto that I've seen before include "Quickly, amigo" , "Immediately, amigo" , "Proton (anag.)" , "Italian telephonic greeting" , "Promptly, without delay" .)