City official tracking the movement of Sterling? (9)

I believe the answer is:

treasurer

'city official' is the definition.
'treasurer' can be an answer for 'official' (I've seen this before). I am unsure of the 'city' bit.

I cannot understand how the rest of the clue works.

(Other definitions for treasurer that I've seen before include "Finance expert" , "Fund manager" , "One of main officers of a society" , "Financial assets manager" , "Money dealer" .)

I've seen this clue in The Guardian.

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