In quiet Irish waterway, jointly use cutter (11)
I believe the answer is:
ploughshare
'cutter' is the definition.
(I've seen this before)
'in quiet irish waterway jointly use' is the wordplay.
'in quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet. I am not sure about the 'in' bit.).
'irish waterway' becomes 'lough' (Irish term for a lake).
'jointly use' becomes 'share' (to share something is to use it with others).
'p'+'lough'+'share'='PLOUGHSHARE'
(Other definitions for ploughshare that I've seen before include "Blade used in farming" , "Cutter" .)