Old man looking up relative point in orbit (5)
I believe the answer is:
apsis
'point in orbit' is the definition.
(astronomical term)
'old man looking up relative' is the wordplay.
'old man' becomes 'pa' (both can mean someone's father).
'looking up' says the letters should be written backwards (in a down clue, letters go up).
'relative' becomes 'sis' (short for sister).
'pa' written backwards gives 'ap'.
'ap'+'sis'='APSIS'
(Other definitions for apsis that I've seen before include "part of church" , "The point of being close" , "point in orbit" , "astronomical point" .)