Government official is bang on shifting river to the west (7)
I believe the answer is:
prefect
'government official' is the definition.
(I know that prefect is a type of government official)
'bang on shifting river to the west' is the wordplay.
I cannot quite understand how this works, but
'on' could be 're' ('on' can be mean 'with reference to') and 're' is found within the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'is' is the link.
Can you help me to learn more?
(Other definitions for prefect that I've seen before include "Senior pupil with powers" , "Roman magistrate; school pupil" , "Chief admin officer of a district in some countries" , "governor" , "Chief officer (in France) - senior pupil" .)