Bird seen in separate line of hills (9)
I believe the answer is:
partridge
'bird' is the definition.
(partridge is a kind of bird)
'separate line of hills' is the wordplay.
'separate' becomes 'part' (synonyms).
'line of hills' becomes 'ridge'.
'part'+'ridge'='PARTRIDGE'
'seen in' acts as a link.
(Other definitions for partridge that I've seen before include "1st" , "Peculiarly apt girder for the game bird" , "Animal" , "Flighty type" , "Game bird (in a pear tree?)" .)