Leave in mind for one section (12)
I believe the answer is:
departmental
'one section' is the definition.
(I have seen 'section of business' mean 'departmental' so perhaps 'section' could also mean 'departmental')
'leave in mind' is the wordplay.
'leave' becomes 'depart' (departing is a kind of leaving).
'in mind' becomes 'mental' (mental means in the mind).
'depart'+'mental'='DEPARTMENTAL'
'for' is the link.
(Other definitions for departmental that I've seen before include "Of an office" , "Divided into sections" , "Concerned with company's divisions" , "sectioned" , "Organised in units" .)