The old school head is in agreement (3)
I believe the answer is:
yes
'agreement' is the definition.
(I've seen this in another clue)
'the old school head' is the wordplay.
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'head' indicates taking the first letters.
The first letter of 'school' is 's'.
'ye'+'s'='YES'
'is in' acts as a link.
(Other definitions for yes that I've seen before include "Absolutely" , "Validating vote" , "Pro vote" , "Affirmative answer" , "Word of affirmation" .)