Lower hood after onset of shower (5)
I believe the answer is:
scowl
'lower' is the definition.
(scowling is a kind of lowering)
'hood after onset of shower' is the wordplay.
'hood' becomes 'cowl' ('cowl' can be a synonym of 'hood').
'after' is a charade indicator (letters next to each other).
'onset of' suggests taking the first letters.
The first letter of 'shower' is 's'.
'cowl' after 's' is 'SCOWL'.
(Other definitions for scowl that I've seen before include "Look angry" , "Look threatening" , "Glare" , "Glower; frown" , "Angry frown" .)