Onset of storm affected little monkey (5)
I believe the answer is:
scamp
'little monkey' is the definition.
(I've seen this before)
'onset of storm affected' is the wordplay.
'onset of' indicates taking the first letters.
'affected' becomes 'camp' (camp can mean exaggerated or affected).
The first letter of 'storm' is 's'.
's'+'camp'='SCAMP'
(Other definitions for scamp that I've seen before include "Scoundrel" , "Mischief-maker" , "Rogue - perform inadequately" , "Impish child" , "Troublemaker" .)