Certainly, son will follow you (3)
I believe the answer is:
yes
'certainly' is the definition.
(both can be used to answer in the affirmative)
'son will follow you' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'will follow' means one lot of letters go next to another.
'you' becomes 'ye' (historical version of 'you').
's' after 'ye' is 'YES'.
(Other definitions for yes that I've seen before include "Word of agreement" , "Of course" , "Affirmative reply" , "3 across" , "Validating vote" .)