Space is ultimately big and likely endless (3)
I believe the answer is:
gap
'space' is the definition.
(gap is a kind of space)
'ultimately big and likely endless' is the wordplay.
'ultimately' says to take the final letters.
'and' means one lot of letters go next to another.
'likely' becomes 'apt' (similar in meaning).
'endless' means to remove the last letter.
The final letter of 'big' is 'g'.
'apt' with its final letter taken away is 'ap'.
'g'+'ap'='GAP'
'is' acts as a link.
(Other definitions for gap that I've seen before include "A break in something continuous, like a hedge" , "Fissure" , "A break or opening as in a hedge" , "fracture" , "Interval, break" .)