Canon's done exercising — it doesn't take long (10)
I believe the answer is:
nanosecond
'it doesn't take long' is the definition.
(very short period of time)
'canon's done exercising' is the wordplay.
'exercising' indicates an anagram (letters exercise or move around).
'canons'+'done'='canonsdone'
'canonsdone' is an anagram of 'NANOSECOND'.
(Other definitions for nanosecond that I've seen before include "Only a little flash" , "instant?" , "Tiny duration" , "A very short interval of time" , "very brief time" .)