Son and dad, covered in blood, become furious (2,5)
I believe the answer is:
go spare
'become furious' is the definition.
'son and dad covered in blood' is the wordplay.
'son' becomes 's'.
'and' says to put letters next to each other.
'dad' becomes 'pa' (synonyms).
'covered in' is an insertion indicator.
'blood' becomes 'gore' (gore is a kind of blood).
's'+'pa'='spa'
'spa' put inside 'gore' is 'GO SPARE'.
(Other definitions for go spare that I've seen before include "rage" , "get very angry" , "Be unused" , "Blow up" , "Become distraught" .)