The writer’s spiteful comments about the City may be foolish (8)
I believe the answer is:
imbecile
'foolish' is the definition.
(thesaurus)
'the writer's spiteful comments about the city' is the wordplay.
'the writer's' becomes 'im' (both could mean 'the crossword setter is').
'spiteful comments' becomes 'bile' (I am not sure about this - if you are sure you should believe this answer much more).
'about' means one lot of letters goes inside another.
'the city' becomes 'ec' (the City of London postcode).
'bile' placed around 'ec' is 'becile'.
'im'+'becile'='IMBECILE'
'may be' acts as a link.
Can you help me to learn more?
(Other definitions for imbecile that I've seen before include "Silly person or idiot" , "fool" , "Moron or cretin" , "Sap" , "Numbskull" .)