Difficult to walk when energy is limited (5)
I believe the answer is:
steep
'difficult' is the definition.
(as in a steep climb)
'walk when energy is limited' is the wordplay.
I cannot really see how this works, but
'energy' could be 'e' (physics symbol as in E=mc2) and 'e' is present in the answer.
'walk' could be 'step' (I've seen this in another clue) and 'step' is present in the remaining letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'to' is the link.
Can you help me to learn more?
(Other definitions for steep that I've seen before include "Lofty" , "Sharply inclined" , "Rapidly rising" , "Rising sharply" , "Sheer, precipitous" .)