Terribly sore, but fine in the end admittedly (2,2,4)
I believe the answer is:
to be sure
'admittedly' is the definition.
(I've seen this before)
'terribly sore but fine in the end' is the wordplay.
'terribly' indicates anagramming the letters.
'in the end' indicates one should take the final letters.
The last letter of 'fine' is 'e'.
'sore'+'but'='sorebut'
'sorebut' anagrammed gives 'tobesur'.
'tobesur'+'e'='TO BE SURE'
(Other definitions for to be sure that I've seen before include "Of course, certainly" , "I admit" , "Admittedly" , "Undoubtedly" , "Without doubt" .)