Crushes a relative (7)

I believe the answer is:

stepson

'relative' is the definition.
(stepson is a kind of relative)

I don't understand how the remainder of the clue works.

(Other definitions for stepson that I've seen before include "second family member" , "Relation, not by blood" , "Male offspring of partner's former marriage" , "younger member of family" , "partner's sprog" .)

I've seen this clue in The Everyman puzzle in The Observer and The Times.

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