Disasters involving upper-class holidays at sea (7)
I believe the answer is:
Here is my best explanation:
'holidays at sea' is the definition.
'disasters involving upper-class' is the wordplay.
'disasters' becomes 'crises'.
'involving' is an insertion indicator.
'upper-class' becomes 'u' ('u' can be similar in meaning to 'upper-class').
'crises' placed around 'u' is 'CRUISES'.
(Another definition for cruises that I've seen is "Holidays on water".)