American power breaks transmitter: one hangs up (9)
I believe the answer is:
suspender
'one hangs up' is the definition.
(someone who suspends things or hangs them up)
'american power breaks transmitter' is the wordplay.
'american' becomes 'us' (abbreviation for United States).
'power' becomes 'p' (symbol in physics).
'breaks' indicates putting letters inside.
'transmitter' becomes 'sender' (synonyms).
'us'+'p'='usp'
'usp' put within 'sender' is 'SUSPENDER'.
(Other definitions for suspender that I've seen before include "hangman?" , "Strap to hold up a stocking" , "One that supports stocking" , "Supporter" , "Stocking-holding strap" .)