Day One absorbed by some spinning Martian satellite (6)
I believe the answer is:
deimos
'martian satellite' is the definition.
(I know this)
'day one absorbed by some spinning' is the wordplay.
'day' becomes 'd' (abbreviation for day).
'one' becomes 'i' (Roman numeral).
'absorbed by' means one lot of letters goes inside another.
'spinning' says the letters should be written backwards.
'i' inserted inside 'some' is 'somie'.
'somie' reversed gives 'eimos'.
'd'+'eimos'='DEIMOS'
(Other definitions for deimos that I've seen before include "Moon of Mars" , "Satellite" .)