Unduly sentimenta,l though not about son being involved in river disaster (9)
I believe the answer is:
cataclysm
'disaster' is the definition.
(I know that disaster can be written as cataclysm)
'unduly sentimental though not about son being involved in river' is the wordplay.
I cannot quite understand how this works, but
'about' could be 'ca' (circa) and 'ca' is found in the answer.
'son' could be 's' (genealogical abbreviation for son) and 's' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for cataclysm that I've seen before include "Event resulting in great loss and misfortune" , "Sudden violent change in the earth's surface" , "A violent upheaval or disaster" , "Disastrous upheaval or flood" , "Deluge: upheaval" .)