City built from rocks during the French period mostly (9)
I believe the answer is:
leicester
'city' is the definition.
(I know that city can be written as Leicester)
'rocks during the french period mostly' is the wordplay.
'rocks' becomes 'ice' (both can informally mean diamonds).
'during' is an insertion indicator.
'the french' becomes 'les' (plural form of 'the' in French).
'period mostly' becomes 'ter' (I am not sure about this - if you are sure you should believe this answer much more).
'ice' put into 'les' is 'leices'.
'leices'+'ter'='LEICESTER'
'built from' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for leicester that I've seen before include "Elizabeth the first's favourite" , "East Midlands city, home of the National Space Centre" , "Midland city" , "At Richmond Palace a favourite" , "City; orange cheese" .)