Arrived at firm — head of accounts encountered inside (4,2)
I believe the answer is:
came to
'arrived at' is the definition.
(to come to a place is to arrive there)
'firm head of accounts encountered inside' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'head of' indicates taking the first letters.
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'inside' is an insertion indicator.
The initial letter of 'accounts' is 'a'.
'a'+'met'='amet'
'co' going around 'amet' is 'CAME TO'.
(Other definitions for came to that I've seen before include "Reached" , "In the main, stopped" .)