Cotton pod's smell mellow in pith (4)
I believe the answer is:
boll
'cotton pod's' is the definition.
(I've seen this in another clue)
'smell mellow in pith' is the wordplay.
'smell' becomes 'B.O.' (abbreviation for Body Odour).
'mellow in pith' becomes 'll' (I can't explain this - if you can you should give a lot more credence to this answer).
'bo'+'ll'='BOLL'
Can you help me to learn more?
(Other definitions for boll that I've seen before include "Some grain" , "swelling" , "Round seed capsule, as in cotton" .)