Okay you old saphead! (3)
I believe the answer is:
yes
'okay' is the definition.
(I've seen this in another clue)
'you old saphead' is the wordplay.
'you old' becomes 'ye' (historical version of 'you').
'saphead' becomes 's' (head letter of 'sap').
'ye'+'s'='YES'
(Other definitions for yes that I've seen before include "Yea" , "Certainly!" , "Word of affirmation" , "3 across" , "I agree" .)