Natural phenomenon that's split a party? (7)
I believe the answer is:
tornado
'natural phenomenon' is the definition.
(tornado is a kind of natural phenomenon)
'split a party?' is the wordplay.
'split' becomes 'torn' (I've seen this in other clues).
'party?' becomes 'do' (do is a kind of party).
'torn'+'a'+'do'='TORNADO'
'that's' is the link.
(Other definitions for tornado that I've seen before include "Severe wind" , "Cyclone, twister" , "RAF fighter; whirlwind" , "Donator of fierce wind" , "threat" .)