Glow from heavenly body in marital bed occasionally (6)
I believe the answer is:
albedo
'glow from heavenly body' is the definition.
(albedo means light reflected by a celestial body)
'in marital bed occasionally' is the wordplay.
'in' says the answer is hidden in the clue.
'ALBEDO' is hidden in the letters of 'marital bed occasionally'.
(Other definitions for albedo that I've seen before include "Measure of a surface's reflectivity" , "Proportion of light a planet reflects" , "Proportion of astronomers" , "Reflectivity of a surface" , "Reflecting power" .)