Attack head of secondary school (3,2)
I believe the answer is:
set on
'attack' is the definition.
(synonyms)
'head of secondary school' is the wordplay.
'head of' indicates taking the first letters.
'school' becomes 'eton' (famous UK public school).
The initial letter of 'secondary' is 's'.
's'+'eton'='SET ON'
(Other definitions for set on that I've seen before include "Attack" , "Determined" , "Go" .)