Semi-dim friend keeps count with mounting energy (9)
I believe the answer is:
penumbral
'semi-dim' is the definition.
(with partial shadow)
'friend keeps count with mounting energy' is the wordplay.
I cannot quite understand how this works, but
'energy' could be 'pe' (P.E. is an example) and 'pe' is found within the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for penumbral that I've seen before include "Shadowy" , "of shadows" .)