Stars in county's borders tweeted back (6)
I believe the answer is:
cygnus
'stars' is the definition.
(I've seen this before)
'county's borders tweeted back' is the wordplay.
'borders' means to remove the middle letters (only the edge or border letters remain).
'tweeted' becomes 'sung' (I've seen this in another clue).
'back' shows that the letters should be reversed in order.
'county' with its middle removed is 'cy'.
'sung' written backwards gives 'gnus'.
'cy'+'gnus'='CYGNUS'
'in' is the link.
(Other definitions for cygnus that I've seen before include "The Swan seen at night" , "Prominent northern constellation, the Swan" , "Operator" , "Stars" .)