Face up perhaps but not to a challenge (4,4)
I believe the answer is:
back down
'face' is the definition.
The definition and answer can be both to do with motion as well as being verbs in their base form.
Perhaps you can see a link between them that I can't see?
'up perhaps but not to a challenge' is the wordplay.
I cannot really see how this works, but
'perhaps' could be 'back' (I've seen this in other clues) and 'back' is found in the answer.
'not' could be 'o' (I've seen this in other clues) and 'o' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for back down that I've seen before include "Give up claim or withdraw argument" , "Withdraw challenge" , "Concede defeat" , "Withdraw (aggressive) claim" , "Withdraw opinion or claim" .)