Novel ending in marriage, then master's marital affair starts (4)
I believe the answer is:
emma
'novel' is the definition.
(I've seen this before)
'ending in marriage then master's marital affair starts' is the wordplay.
'ending in' suggests the final letters.
'then' means one lot of letters go next to another.
'starts' indicates taking the first letters.
The last letter of 'marriage' is 'e'.
The initial letters of 'masters marital affair' is 'mma'.
'e'+'mma'='EMMA'
(Other definitions for emma that I've seen before include "Austen character" , "Eponymous heroine" , "Victorian prime minister" , "novelist's work" , "Titled heroine" .)