The large knots restricting piano wire (9)
I believe the answer is:
telegraph
'wire' is the definition.
(synonyms)
'the large knots restricting piano' is the wordplay.
'knots' is an anagram indicator (I've seen 'knotted' mean this).
'restricting' means one lot of letters goes inside another.
'piano' becomes 'p' (musical abbreviation).
'the'+'large'='thelarge'
'thelarge' with letters rearranged gives 'telegrah'.
'telegrah' going around 'p' is 'TELEGRAPH'.
(Other definitions for telegraph that I've seen before include "Electrical transmission of coded signals" , "Device for transmitting information over distance" , "A scoreboard" , "Daily" , "System of communication over wires" .)