Mark again needs relative -- second son (7)

I believe the answer is:

relabel

'mark again' is the definition.
('label' can be a synonym for 'mark' so perhaps 'relabel' is OK for 'mark again')

'relative second son' is the wordplay.
'relative' becomes 'rel' (mathematical abbreviation).
'second son' becomes 'abel' (I've seen this before).
'rel'+'abel'='RELABEL'

'needs' acts as a link.

(Another definition for relabel that I've seen is " Put a new tag".)

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