Head girl provides spice (6)
I believe the answer is:
nutmeg
'spice' is the definition.
(I know that Nutmeg is a type of spice)
'head girl' is the wordplay.
'head' becomes 'nut' (informal term for somebody's head).
'girl' becomes 'meg' ().
'nut'+'meg'='NUTMEG'
'provides' acts as a link.
(Other definitions for nutmeg that I've seen before include "it's used by bakers" , "Hard seed used as seasoning" , "put the ball through" , "Fragrant tropical seed ground as spice" , "Kernel and girl for a spice" .)