Tons in river added to maintain pressure without planning (9)
I believe the answer is:
extempore
'without planning' is the definition.
(extempore can mean improvised)
'tons in river added to maintain pressure' is the wordplay.
I cannot really see how this works, but
'tons' could be 't' and 't' is located in the answer.
'river' could be 'exe' (River Exe) and 'exe' is found within the leftover letters.
'pressure' could be 'p' (abbreviation) and 'p' is present in the answer.
'added' could be 'more' (similar in meaning) and 'more' is found in the remaining letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for extempore that I've seen before include "With no notes" , "Dashed off" , "on spur of the moment" , "Spoken off the cuff" , "on the spot" .)