Son, mischievous, inconsiderate toward others (7)
I believe the answer is:
selfish
'inconsiderate toward others' is the definition.
'son mischievous' is the wordplay.
'son' becomes 's'.
'mischievous' becomes 'elfish' ('elfish' can be a synonym of 'mischievous').
's'+'elfish'='SELFISH'
(Other definitions for selfish that I've seen before include "Egotistical" , "Concerned chiefly with one's own interests" , "Egocentric - mean" , "without consideration" , "inconsiderate" .)