Hammer-based instruments (6)
I believe the answer is:
pianos
'hammer-based instruments' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for pianos that I've seen before include "Chordophones with keys" , "Hammer and strings instruments" , "Keyboard instruments" , "Hammer-based instruments" , "Musical instruments like Steinways" .)