Son in row over another exam (5)
I believe the answer is:
resit
'another exam' is the definition.
(sitting an exam again)
'son in row over' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'in' is an insertion indicator.
'row' becomes 'tier' (a tier is a row of things).
'over' says the letters should be written in reverse.
's' inserted into 'tier' is 'tiser'.
'tiser' written backwards gives 'RESIT'.
(Other definitions for resit that I've seen before include "take another exam" , "Enquiring intrusively (into person's affairs)" , "Take examination again" , "second try at test" , "Exam taken again" .)