Certainly the first person heard? (3)
I believe the answer is:
aye
'certainly' is the definition.
(both can be used to answer in the affirmative)
'first person heard?' is the wordplay.
'first person' becomes 'I' (first-person pronoun).
'heard?' shows a homophone (sound like).
'i' is a homophone of 'AYE'.
'the' is the link.
(Other definitions for aye that I've seen before include "An affirmative vote" , "Indeed" , "Affirmative answer" , "Positive voting response" , "Always" .)