Orbital minimum (7)
I believe the answer is:
perigee
'orbital minimum' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for perigee that I've seen before include "Point of orbit nearest earth" , "Point of closest approach" , "Lowest orbit point" , "where orbit's lowest" , "Point where the Moon is closest to the Earth" .)