Witnesses aim to break close relatives? (7,5)
I believe the answer is:
siamese twins
'close relatives?' is the definition.
(conjoined twins)
'witnesses aim to break' is the wordplay.
'to break' indicates an anagram.
'witnesses'+'aim'='witnessesaim'
'witnessesaim' is an anagram of 'SIAMESE TWINS'.
(Other definitions for siamese twins that I've seen before include "Siblings joined at birth" , "We're United" , "Physically joined babies" .)