Monkey affected by sulphur (5)
I believe the answer is:
scamp
'monkey' is the definition.
(both can mean a mischievous child)
'affected by sulphur' is the wordplay.
'affected' becomes 'camp' (camp can mean exaggerated or affected).
'by' means one lot of letters go next to another.
'sulphur' becomes 'S' (synonyms).
'camp' put after 's' is 'SCAMP'.
(Other definitions for scamp that I've seen before include "Mischief-maker" , "villain" , "Impish child" , "pickle" , "Impish youngster" .)