Power seized by hardened chief (6)
I believe the answer is:
staple
'chief' is the definition.
(I've seen this before)
'power seized by hardened' is the wordplay.
'power' becomes 'p' (symbol in physics).
'seized by' means one lot of letters goes inside another.
'hardened' becomes 'stale' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'p' going inside 'stale' is 'STAPLE'.
Can you help me to learn more?
(Other definitions for staple that I've seen before include "Paper fastener - basic foodstuff" , "One securing hasp; principal" , "The food one normally eats" , "U-shaped wire to fasten papers together" , "Basic commodity and paper fastener" .)