Upset the City when American hosts fine "do" (7)
I believe the answer is:
suffice
'do' is the definition.
('suffice' can be a synonym of 'do')
'upset the city when american' is the wordplay.
'upset' is a reversal indicator.
'the city' becomes 'eci' (ECf is the City of London postcode).
'when' becomes 'ff' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'american' becomes 'us'.
'eci'+'ff'+'us'='eciffus'
'eciffus' in reverse letter order is 'SUFFICE'.
'hosts fine' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
(Other definitions for suffice that I've seen before include "Be enough" , "Serve - do" , "Be adequate" .)