Huge blow to a party supporting Royal Navy (7)
I believe the answer is:
tornado
'huge blow' is the definition.
'tornado' can be an answer for 'blow' (a tornado blows around). I am not sure about the 'huge' bit.
'to a party supporting royal navy' is the wordplay.
'party' becomes 'do' (do can informally mean a party).
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'royal navy' becomes 'RN' (abbreviation for Royal Navy).
'a'+'do'='ado'
'ado' put after 'rn' is 'rnado'.
'to'+'rnado'='TORNADO'
Can you help me to learn more?
(Other definitions for tornado that I've seen before include "Destructive weather" , "It's a big blow" , "Donator (anag.)" , "A major blow" , "Hurricane" .)