Hum first of tunes in flop (5)
I believe the answer is:
stink
'hum' is the definition.
(I've seen this before)
'first of tunes in flop' is the wordplay.
'first of' indicates taking the first letters.
'in' indicates putting letters inside.
'flop' becomes 'sink'.
The first letter of 'tunes' is 't'.
't' going within 'sink' is 'STINK'.
(Other definitions for stink that I've seen before include "Bad smell" , "Unpleasant smell" , "Foul smell" , "Hum" , "song and dance" .)