Organ in a socket (3)
I believe the answer is:
eye
'organ in a socket' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eye that I've seen before include "'Cast a cold ... on life, on death' (Yeats)" , "Viewer" , "Storm-centre" , "May be private" , "20 [OBSERVE]" .)