Stop and help returning father and son (8)
I believe the answer is:
diapason
'stop' is the definition.
(I've seen this before)
'help returning father and son' is the wordplay.
'help' becomes 'aid' (synonyms).
'returning' says the letters should be written backwards.
'father' becomes 'pa' (pa can mean dad or father).
'and' says to put letters next to each other.
'aid' in reverse letter order is 'dia'.
'dia'+'pa'+'son'='DIAPASON'
'and' acts as a link.
(Other definitions for diapason that I've seen before include "Othello's enemy" , "Organ stop; sad piano (anag.)" , "tuning fork" , "Entire range of musical instrument or voice" .)