One serving in the military holds Mass for city (5)
I believe the answer is:
parma
'for city' is the definition.
(I have seen 'Italian city' mean 'parma' so perhaps 'city' could also mean 'parma')
'one serving in the military holds mass' is the wordplay.
'one serving in the military' becomes 'para' (I can't justify this - if you can you should give a lot more credence to this answer).
'holds' indicates putting letters inside.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'para' placed around 'm' is 'PARMA'.
Can you help me to learn more?
(Other definitions for parma that I've seen before include "Italian city known for ham and violets" , "Italian city famous for cheese and ham" , "Italian city noted for ham" , "Here in Italy" , "Source of ham and cheese" .)