Agitator from firm's foremost US site (7)
I believe the answer is:
fusspot
'agitator' is the definition.
(term for a fussy person)
'firm's foremost us site' is the wordplay.
'foremost' suggests taking the first letters.
'site' becomes 'spot' (I've seen this in another clue).
The initial letter of 'firms' is 'f'.
'f'+'us'+'spot'='FUSSPOT'
'from' is the link.
(Other definitions for fusspot that I've seen before include "Person hard to please" , "bit of a worrier" , "Nervously agitating type" , "song-and-dance man?" , "I worry" .)