Old city with mass in work showing disquiet (7)
I believe the answer is:
turmoil
'disquiet' is the definition.
(thesaurus)
'old city with mass in work' is the wordplay.
'old city' becomes 'ur' (ancient city state).
'with' means one lot of letters go next to another.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'in' is an insertion indicator.
'work' becomes 'toil' (toiling is a kind of working**).
'ur'+'m'='urm'
'urm' put into 'toil' is 'TURMOIL'.
'showing' acts as a link.
(Other definitions for turmoil that I've seen before include "Agitation, confusion" , "Hubbub" , "Wild disorder" , "Disquiet" , "Pandemonium" .)