Time and space mostly set up (3)
I believe the answer is:
era
'time' is the definition.
(era is a kind of time)
'space mostly set up' is the wordplay.
'space' becomes 'area' (I've seen this before).
'mostly' means to remove the last letter.
'set up' says the letters should be written in reverse.
'area' with its last letter taken off is 'are'.
'are' backwards is 'ERA'.
'and' is the link.
(Other definitions for era that I've seen before include "Specific time" , "Important time" , "day" , "Long historic period" , "8" .)