Married cheat dashing first daughter’s heart (6)

I believe the answer is:

middle

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'heart' is the definition.
(I know that heart can be written as middle)

I can't explain the remainder of the clue.

(Other definitions for middle that I've seen before include "One of 21 [HEARTS]" , "Equally distant from the extremes" , "Intermediate" , "Centre or core" , "Centre point" .)

I've seen this clue in The Guardian.

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