Problem with son riding horse (4)
I believe the answer is:
snag
'problem' is the definition.
(snag can mean a problem or difficulty)
'son riding horse' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'riding' means one lot of letters go next to another (in a down clue, some letters go on top of others).
'horse' becomes 'nag' (term for an old horse).
's'+'nag'='SNAG'
'with' acts as a link.
(Other definitions for snag that I've seen before include "Hidden obstacle or drawback" , "Tear - catch" , "Catch -- problem" , "Rip or tear" , "Sharp protuberance; catch" .)