The writer loses first wife, beheaded in the river (7)

'the river' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'the writer loses first wife beheaded' is the wordplay.
'the writer' becomes 'I' (the person who is writing these clues).
'loses first' becomes 'l' (1st letter of 'loses').
'wife' becomes 'missus' (informal term for someone's wife).
'beheaded' means to remove the first letter (the head of the word is removed).
'missus' with its first letter taken off is 'issus'.
'i'+'l'+'issus'='ILISSUS'

'in' acts as a link.

Can you help me to learn more?