Bath leads to this enormous flood (5)
I believe the answer is:
spate
'flood' is the definition.
(synonyms)
'bath leads to this enormous' is the wordplay.
'bath' becomes 'spa' (I've seen this before).
'leads to' indicates taking the first letters.
The first letters of 'this enormous' is 'te'.
'spa'+'te'='SPATE'
(Other definitions for spate that I've seen before include "Deluge" , "Flood; sudden burst" , "Fast outpouring" , "Sudden river flood" , "Rush, flood (of river)" .)