Check son getting into coach again (8)
I believe the answer is:
restrain
'check' is the definition.
(restraining is a kind of checking)
'son getting into coach again' is the wordplay.
'son getting' becomes 's' (I've seen this in other clues).
'into' is an insertion indicator.
'coach again' becomes 'retrain' ('train' can be a synonym for 'coach' so perhaps 'retrain' is OK for 'coach again').
's' placed inside 'retrain' is 'RESTRAIN'.
(Other definitions for restrain that I've seen before include "In arrest (anagram)" , "Stop" , "use handcuffs?" , "Keep in check" , "Control - check" .)