Weak note isn’t sensed by the ear when another one interrupts (5-7)
I believe the answer is:
faint-hearted
'weak' is the definition.
(thesaurus)
'note isn't sensed by the ear when another one interrupts' is the wordplay.
'note' becomes 'f' (musical note).
'isn't' becomes 'aint' (non-standard version of 'isn't').
'sensed by the ear' becomes 'heard'.
'when another' becomes 'te' (I can't explain this - if you can you should give a lot more credence to this answer).
'one interrupts' is an insertion indicator.
'heard' placed around 'te' is 'hearted'.
'f'+'aint'+'hearted'='FAINT-HEARTED'
(Other definitions for faint-hearted that I've seen before include "chicken" , "Timorous - find a theatre (anag)" , "Timid" .)