School garden party — head not required to take part (7)
I believe the answer is:
compete
'take part' is the definition.
'school garden party head not required' is the wordplay.
'school' becomes 'comp' (short for comprehensive school).
'garden party' becomes 'fete' (I've seen this before).
'head not required' means to remove the first letter.
'fete' with its initial letter removed is 'ete'.
'comp'+'ete'='COMPETE'
'to' is the link.
(Other definitions for compete that I've seen before include "Strive (against others)" , "Engage in a contest" , "Struggle for victory" , "Vie, contest" , "Take part (in race)" .)