Greetings to Royal Marines: violent shower? (9)
I believe the answer is:
hailstorm
'violent shower?' is the definition.
(I've seen this before)
'greetings to royal marines' is the wordplay.
'greetings' becomes 'hails' (hail is a kind of greeting).
'royal marines' becomes 'RM' (abbreviation).
'hails'+'to'+'rm'='HAILSTORM'
(Other definitions for hailstorm that I've seen before include "rough weather" , "Fall of ice" , "Wintry bad weather" , "when Sky drops The Stones?" , "It brings stones from the sky" .)