Imp's sec in shed (5)
I believe the answer is:
demon
'imp's' is the definition.
(I've seen this before)
'sec in shed' is the wordplay.
'sec' becomes 'mo' (both can mean a small amount of time).
'in' is an insertion indicator.
'shed' becomes 'den' (I am not sure about this - if you are sure you should believe this answer much more).
'mo' inserted into 'den' is 'DEMON'.
Can you help me to learn more?
(Other definitions for demon that I've seen before include "Evil spirit or cruel person" , "Monster" , "Wizard" , "Inspiring force" , "Expert" .)