Initially expecting a more serious person from Boston, say (9)
I believe the answer is:
easterner
'person from boston say' is the definition.
(person from the east of the USA)
'initially expecting a more serious' is the wordplay.
'initially' indicates taking the first letters.
'more serious' becomes 'sterner' (I've seen this before).
The initial letter of 'expecting' is 'e'.
'e'+'a'+'sterner'='EASTERNER'
(Other definitions for easterner that I've seen before include "one from Boston, say" , "New Yorker perhaps" , "Resident of Connecticut, perhaps" , "Asian" , "New Englander, for instance" .)