Son’s shunning fast food (4)
I believe the answer is:
tuck
'food' is the definition.
(thesaurus)
'son's shunning fast' is the wordplay.
'shunning' suggests deleting specific letters.
'fast' becomes 'stuck'.
'stuck' with 's' removed is 'TUCK'.
(Other definitions for tuck that I've seen before include "legendary friar" , "Fold - friar!" , "Fold into small space" , "Eats" , "Fold - grub" .)