Splits rent with son (5)
I believe the answer is:
tears
'splits' is the definition.
('tear' can be a synonym of 'split')
'rent with son' is the wordplay.
'rent' becomes 'tear' (synonyms).
'with' means one lot of letters go next to another.
'son' becomes 's' (genealogical abbreviation for son).
'tear'+'s'='TEARS'
(Other definitions for tears that I've seen before include "Signs of unhappiness" , "Rents" , "See 12" , "signs of emotion" , "Rips; drops" .)