He's around when there's some madcap mischief (3)
I believe the answer is:
imp
'some madcap mischief' is the definition.
'imp' can be an answer for 'mischief' (thesaurus). I'm not sure about the 'some madcap' bit.
'he's around when' is the wordplay.
'he' becomes 'vic' (short for Victor).
'around' indicates the central letters.
'when' becomes 'mp' (I am not sure about this - if you are sure you should believe this answer much more).
The central letter of 'vic' is 'i'.
'i'+'mp'='IMP'
'there's' acts as a link.
Can you help me to learn more?
(Other definitions for imp that I've seen before include "A mischievous little devil" , "Little troublemaker" , "Scallywag" , "Little mischief-maker" , "A little devil of mischief" .)