Tone up, principally by running and callisthenic exercises (5)
I believe the answer is:
brace
'tone up' is the definition.
'principally by running and callisthenic exercises' is the wordplay.
'principally' says to take the initial letters.
The initial letters of 'by running and callisthenic exercises' is 'BRACE'.
(Other definitions for brace that I've seen before include "hand tool" , "Stiffen; pair" , "A pair as of game birds" , "double" , "Pair - wood-boring tool" .)