Overwhelming rush of pain besetting naval invalid (9)
I believe the answer is:
avalanche
'overwhelming rush' is the definition.
(rush of snow)
'pain besetting naval invalid' is the wordplay.
'pain' becomes 'ache' (ache is a kind of pain).
'besetting' is an insertion indicator.
'invalid' indicates anagramming the letters (an incorrect or invalid spelling).
'naval' is an anagram of 'valan'.
'ache' enclosing 'valan' is 'AVALANCHE'.
'of' acts as a link.
(Other definitions for avalanche that I've seen before include "sudden rush" , "Large snow or rock fall" , "Large fall of snow or rocks" , "Slide of large amounts of snow or stone" , "Deluge" .)