Exactly the place where a crank is powerless (4,6)
I believe the answer is:
dead centre
'a crank is powerless' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'exactly the place' is the wordplay.
'exactly' becomes 'dead' (eg in phrases like 'you're dead right').
'the place' becomes 'centre' (I can't justify this - if you can you should believe this answer much more).
'dead'+'centre'='DEAD-CENTRE'
'where' acts as a link.
Can you help me to learn more?
(Other definitions for dead centre that I've seen before include "cemetery?" , "Position when engine crank and connecting-rod are in line" , "Smack in the middle" , "bullseye" , "Right in the middle" .)