City's cruel guards turned old harbour ablaze (12)
I believe the answer is:
metropolitan
'city's' is the definition.
(thesaurus)
'cruel guards turned old harbour ablaze' is the wordplay.
'cruel' becomes 'mean' (I've seen this in another clue).
'guards' is an insertion indicator (inserted letters are guarded).
'turned old harbour' becomes 'tropo' (I can't explain this - if you can you should give a lot more credence to this answer).
'ablaze' becomes 'lit' (I've seen this in another clue).
'tropo'+'lit'='tropolit'
'mean' enclosing 'tropolit' is 'METROPOLITAN'.
Can you help me to learn more?
(Other definitions for metropolitan that I've seen before include "Someone coming from Paris, say" , "Archbishop" , "Of a large city -- London Underground line" , "senior bishop?" , "Relating to a large city" .)