Stop to assist returning dad and lad (8)
I believe the answer is:
diapason
'stop' is the definition.
(thesaurus)
'assist returning dad and lad' is the wordplay.
'assist' becomes 'aid' ('aid' can be a synonym of 'assist').
'returning' is a reversal indicator.
'dad' becomes 'pa' ('pa' can be a synonym of 'dad').
'and' is a charade indicator (letters next to each other).
'lad' becomes 'son'.
'aid' written backwards gives 'dia'.
'dia'+'pa'+'son'='DIAPASON'
'to' is the link.
(Other definitions for diapason that I've seen before include "Entire range of musical instrument or voice" , "Othello's enemy" , "Organ stop; sad piano (anag.)" , "tuning fork" .)