Officer gets supporters positioned round hilltop (5,3)
I believe the answer is:
brass hat
'officer' is the definition.
(brass hat is a kind of officer)
'supporters positioned round hilltop' is the wordplay.
'supporters' becomes 'bras' (I've seen this in other clues).
'positioned' becomes 'sat' (sitting is a kind of positioning).
'round' is an insertion indicator.
'hilltop' becomes 'h' (top of the word 'hill').
'sat' enclosing 'h' is 'shat'.
'bras'+'shat'='BRASS HAT'
'gets' acts as a link.
(Other definitions for brass hat that I've seen before include "High-ranking officer in armed forces (colloq.)" , "High-ranking army officer" , "Senior mil. officer (slang)" , "Top-ranking officer" , "High-ranking military officer" .)