Son, daughter, holding article that’s found on beach (4)
I believe the answer is:
sand
'found on beach' is the definition.
'son daughter holding article' is the wordplay.
'son' becomes 's'.
'daughter' becomes 'd'.
'holding' is an insertion indicator.
'article' becomes 'an' (indefinite article).
's'+'d'='sd'
'sd' placed around 'an' is 'SAND'.
'that's' is the link.
(Other definitions for sand that I've seen before include "Content of hourglass" , "George -, nineteenth century French novelist" , "Beach constituent" , "Parts of beach" , "Yellow-brown colour" .)