Support for head of police (4)
I believe the answer is:
prop
'support' is the definition.
(prop is a kind of support)
'for head of police' is the wordplay.
'for' becomes 'pro' (I've seen this before).
'head of' says to take the initial letters.
The initial letter of 'police' is 'p'.
'pro'+'p'='PROP'
(Other definitions for prop that I've seen before include "Stay" , "Set piece" , "Shore for 4 down" , "Support (a rugby player?)" , "Back up" .)