Hasten start of punt in river before change in tide (8)
I believe the answer is:
expedite
'hasten' is the definition.
('expedite' can be a synonym of 'hasten')
'start of punt in river before change in tide' is the wordplay.
'start of' says to take the initial letters.
'in' is an insertion indicator.
'river' becomes 'exe' (I've seen this before**).
'before' means one lot of letters go next to another.
'change in' is an anagram indicator.
The initial letter of 'punt' is 'p'.
'tide' is an anagram of 'dite'.
'exe'+'dite'='exedite'
'p' going into 'exedite' is 'EXPEDITE'.
(Other definitions for expedite that I've seen before include "Hasten, facilitate" , "Hasten (process)" , "Rush" , "Cause to be done more quickly" , "Assist the progress of" .)