Imperturbable admission of inertia? (9)
I believe the answer is:
impassive
'imperturbable' is the definition.
(synonyms)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for impassive that I've seen before include "poker-faced" , "Unconcerned" , "Not showing emotions in face" , "unmoved" , "Inscrutable, not showing emotion" .)