Head of a government department (8)
I believe the answer is:
minister
'head of a government department' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for minister that I've seen before include "Holder of office in government or church" , "Merit sin (anag.)" , "Parson" , "Clergyman - government member" , "Government or church officer" .)