Onset of shower affected little monkey (5)
I believe the answer is:
scamp
'little monkey' is the definition.
(I've seen this before)
'onset of shower affected' is the wordplay.
'onset of' indicates taking the first letters.
'affected' becomes 'camp' (I've seen this before).
The first letter of 'shower' is 's'.
's'+'camp'='SCAMP'
(Other definitions for scamp that I've seen before include "Scoundrel" , "A young rascal misbehaving in camps" , "Impish youngster" , "Little Devil" , "little monkey" .)