Mail worker’s position at head of staff (7)
I believe the answer is:
postman
'mail worker's' is the definition.
(someone who delivers mail)
'position at head of staff' is the wordplay.
'position' becomes 'post' (post is a kind of position).
'at head of' says to put letters next to each other (some letters go in front of others).
'staff' becomes 'man' (as in staffing or manning a particular place).
'post'+'man'='POSTMAN'
(Other definitions for postman that I've seen before include "TV's Pat, for instance" , "Letter deliverer" , "No stamp needed, stangely, for this job" , "Chap with letters" , "Male mail carrier" .)