Moon, mostly, keeps spinning around in space (6)
I believe the answer is:
lacuna
'space' is the definition.
(thesaurus)
'moon mostly keeps spinning around' is the wordplay.
'moon' becomes 'lunar'.
'mostly' means to remove the last letter.
'keeps' is an insertion indicator.
'spinning' says the letters should be written in reverse.
'around' becomes 'ca' (circa).
'ca' backwards is 'ac'.
'lunar' with its last letter taken off is 'luna'.
'luna' placed around 'ac' is 'LACUNA'.
'in' is the link.
(Other definitions for lacuna that I've seen before include "Gap or hiatus" , "Blank gap or missing part" , "A gap, maybe in a book" , "Hiatus - cavity" , "Gap or missing portion" .)