Source pair that turn out to be unstable (10)
I believe the answer is:
precarious
'unstable' is the definition.
(I know that unstable can be written as precarious)
'source pair that turn out' is the wordplay.
'that turn out' indicates an anagram.
'source'+'pair'='sourcepair'
'sourcepair' with letters rearranged gives 'PRECARIOUS'.
'to be' acts as a link.
(Other definitions for precarious that I've seen before include "having difficulty remaining upright" , "Hairy" , "Highly insecure" , "Uncertain; likely to fall" , "Not having a steady position" .)