Domestic servant’s husband, crazy about one on river (9)
I believe the answer is:
housemaid
'domestic servant's' is the definition.
'housemaid' can be an answer for 'servant's' (housemaid is a kind of servant). I'm unsure of the 'domestic' bit.
'husband crazy about one on river' is the wordplay.
'husband' becomes 'house husband' (house husband is a kind of husband).
'crazy about one on river' becomes 'maid' (I can't justify this - if you can you should give a lot more credence to this answer).
'house'+'maid'='HOUSEMAID'
Can you help me to learn more?
(Other definitions for housemaid that I've seen before include "Help" , "scrubber" , "girl with bad knee?" , "menial role" .)