Space program ambitious, Gemini initially set back (3)
I believe the answer is:
gap
'space' is the definition.
(I know that gap is a type of gap)
'program ambitious gemini initially set back' is the wordplay.
'initially' says to take the initial letters.
'set back' is a reversal indicator.
The first letters of 'program ambitious gemini' is 'pag'.
'pag' in reverse letter order is 'GAP'.
(Other definitions for gap that I've seen before include "Room" , "Fissure" , "Break, hole, space" , "Break in continuity" , "Small opening" .)