Go with guys foremost in the field (10)
I believe the answer is:
department
'field' is the definition.
(department can mean a field or area of work)
'go with guys foremost in the' is the wordplay.
'go' becomes 'depart' (synonyms).
'with' means one lot of letters go next to another.
'guys' becomes 'men' (guy can mean a man).
'foremost in' suggests taking the first letters.
The initial letter of 'the' is 't'.
'depart'+'men'+'t'='DEPARTMENT'
(Other definitions for department that I've seen before include "not the whole company" , "Division of a large organisation" , "Organisational division" , "Division of company" , "Ministry" .)