That's frightening about fan's collapse (4,4)
I believe the answer is:
keel over
'collapse' is the definition.
(I've seen this before)
'that's frightening about fan's' is the wordplay.
'that's frightening' becomes 'eek' (expression of fear).
'about' says the letters should be written in reverse.
'fan' becomes 'lover' (synonyms).
'eek' back-to-front is 'kee'.
'kee'+'lover'='KEEL OVER'
(Other definitions for keel over that I've seen before include "Suddenly collapse" , "Pass out" , "Fall, as in a faint" , "Capsize" .)