Say why more attractive new partner was sought? (7)
I believe the answer is:
explain
'say why more attractive new partner' is the definition.
'explain' can be an answer for 'say' (explaining is a kind of saying). I am not certain of the 'why more attractive new partner' bit.
'was sought?' is the wordplay.
'was' becomes 'ex' (I've seen this in other clues).
'sought?' becomes 'plain' (I can't explain this - if you can you should give a lot more credence to this answer).
'ex'+'plain'='EXPLAIN'
Can you help me to learn more?
(Other definitions for explain that I've seen before include "Give reason for" , "Give reasons for" , "Shed light" , "Elucidate, clarify" , "Describe" .)