A bit of exercise? (5)
I believe the answer is:
drill
This clue is a double definition.
'a bit' is the first definition.
(thesaurus)
'exercise?' is the second definition.
(military exercise)
'of' is the link.
(Other definitions for drill that I've seen before include "Exercise or teach through repeated practice" , "School" , "Material - exercise" , "Monkey" , "Boring thing" .)