Maintain altitude so as not to fall behind (4,2)
I believe the answer is:
keep up
'maintain' is the definition.
(I know that maintain can be written as keep up)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for keep up that I've seen before include "Progress at same pace as others, or maintain appearance" , "Stay in contention" , "Match pace or store aloft" , "Don't dawdle!" , "Maintain pace with, don't lag behind" .)