Say why former partner's not attractive (7)

I believe the answer is:

explain

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'say why' is the definition.
'explain' can be an answer for 'say' (explaining is a kind of saying). I am unsure of the 'why' bit.

'former partner's not attractive' is the wordplay.
'former partner' becomes 'ex' (an ex-partner).
'not attractive' becomes 'plain' (I've seen this before).
'ex'+'plain'='EXPLAIN'

Can you help me to learn more?

(Other definitions for explain that I've seen before include "Give reasons for" , "Expound" , "Show the meaning of, make clear" , "Elucidate, make clear" , "Defend" .)

I've seen this clue in The Guardian.

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