Half-sister of Absalom (5)
I believe the answer is:
tamar
'half-sister of absalom' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tamar that I've seen before include "West Country river; a tram (anag.)" , "River in south-west" , "River of SW_England" , "Devon-Cornwall boundary river" , "Plymouth river" .)