Notice wife plunged into warmer stream (9)

I believe the answer is:

headwater

'stream' is the definition.
(I've seen this before)

'notice wife plunged into warmer' is the wordplay.
'notice' becomes 'ad' (both can mean an advertisement).
'wife' becomes 'w' (abbreviation).
'plunged into' means one lot of letters goes inside another.
'warmer' becomes 'heater' (synonyms).
'ad'+'w'='adw'
'adw' inserted inside 'heater' is 'HEADWATER'.

(Another definition for headwater that I've seen is " stream".)

I've seen this clue in The Telegraph.

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