High priest number two holding mass for stricken city (7)
I believe the answer is:
pompeii
'stricken city' is the definition.
(city destroyed by Mount Vesuvius)
'high priest number two holding mass' is the wordplay.
'high priest' becomes 'pope' (leader of the Catholic Church).
'number' becomes 'i' (I is an example).
'two' indicates the doubling of some letters.
'holding' is an insertion indicator.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'i' duplicated is 'ii'.
'pope'+'ii'='popeii'
'popeii' placed around 'm' is 'POMPEII'.
'for' acts as a link.
(Other definitions for pompeii that I've seen before include "Ruined Roman town" , "Roman site" , "Roman city destroyed in 79AD" , "Ancient Italian city" , "Ancient city buried when Vesuvius erupted" .)