Certainly you'll experience pain initially (3)
I believe the answer is:
yep
'certainly' is the definition.
(thesaurus)
'you'll experience pain initially' is the wordplay.
'initially' says to take the initial letters.
The initial letters of 'youll experience pain' is 'YEP'.
(Other definitions for yep that I've seen before include "Affirmative (US colloq.)" , "informal approval" , "Positive reaction" , "Correct" .)