Endless ordeal involving your old heavenly body (6)
I believe the answer is:
tethys
'heavenly body' is the definition.
(Tethys is a moon of Saturn)
'endless ordeal involving your old' is the wordplay.
'endless' means to remove the last letter.
'ordeal' becomes 'test' (a testing situation).
'involving' indicates putting letters inside.
'your old' becomes 'thy' ('thy' historically meant 'your').
'test' with its last letter taken away is 'tes'.
'tes' going around 'thy' is 'TETHYS'.
(Other definitions for tethys that I've seen before include "Moon" , "Wife of Oceanus" , "Old sea" , "goddess" .)