Percussion instrument was hard to get on table (9)
I believe the answer is:
washboard
'percussion instrument' is the definition.
(a washboard can be used as a musical instrument)
'was hard to get on table' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to get on' means one lot of letters go next to another (in a down clue, some letters go above others).
'table' becomes 'board' (board is a kind of table).
'was'+'h'+'board'='WASHBOARD'
(Other definitions for washboard that I've seen before include "A percussion instrument" , "On which to scrub clothes" , "It can be played" , "Skiffle instrument" .)