Open space added to slowly? (8)
I believe the answer is:
unfasten
'open' is the definition.
(unfastening is a kind of opening)
'space added to slowly?' is the wordplay.
'space' becomes 'en' (in printing, a space as wide as a letter 'n').
'added to' means one lot of letters go next to another.
'slowly?' becomes 'unfast' (I am not sure about this - if you are sure you should believe this answer much more).
'en' after 'unfast' is 'UNFASTEN'.
Can you help me to learn more?
(Other definitions for unfasten that I've seen before include "Nun's fate (anag.)" , "Unlock" , "Loosen or open, say seatbelt" , "tie? No" , "Undo" .)