Curtailed the strange monotonous sound (5)
I believe the answer is:
thrum
'monotonous sound' is the definition.
(I've seen this in another clue)
'curtailed the strange' is the wordplay.
'curtailed' means to remove the last letter.
'strange' becomes 'rum' (synonyms).
'the' with its last letter taken away is 'th'.
'th'+'rum'='THRUM'
(Other definitions for thrum that I've seen before include "Drum incessantly" , "tattoo" , "Drum with fingers - bits of coarse yarn" , "Tap a beat with fingers" , "Strum monotonously" .)