Suspect humming head off (4)
I believe the answer is:
iffy
'suspect' is the definition.
(I've seen this before)
'humming head off' is the wordplay.
'humming' becomes 'niffy' (informal term meaning smelly).
'head off' means to remove the first letter.
'niffy' with its initial letter removed is 'IFFY'.
(Other definitions for iffy that I've seen before include "Informally dubious, not definite" , "Dubious, risky (inf.)" , "Unsure" , "Not to be trusted" , "not kosher" .)