Starts to hear strange monotonous sound (5)
I believe the answer is:
thrum
'monotonous sound' is the definition.
(I've seen this in another clue)
'starts to hear strange' is the wordplay.
'starts' suggests taking the first letters.
'strange' becomes 'rum' (similar in meaning).
The initial letters of 'to hear' is 'th'.
'th'+'rum'='THRUM'
(Other definitions for thrum that I've seen before include "Perform monotonously" , "Inexpressively play" , "Repetitive drone - end of weaver's thread" , "Strum, hum or drum!" , "Drum incessantly" .)