Warship's funny story, invading two rivers (9)
I believe the answer is:
destroyer
'warship's' is the definition.
(destroyer is a kind of warship)
'funny story invading two rivers' is the wordplay.
'two rivers' can mean two replacements for 'river'.
'funny' indicates anagramming the letters.
'invading' is an insertion indicator.
'river' becomes 'dee' (River Dee).
'river' becomes 'r'.
'story' with letters rearranged gives 'stroy'.
'stroy' going into 'dee' is 'destroye'.
'destroye'+'r'='DESTROYER'
(Other definitions for destroyer that I've seen before include "Saboteur" , "One scuppers" , "Heavily-armed warship" , "Vandal" , "HMS" .)