Sponge has an unpleasant smell (4)
I believe the answer is:
pong
'an unpleasant smell' is the definition.
'pong' can be an answer for 'smell' (pong can informally mean a bad smell). I am not certain of the 'an unpleasant' bit.
'sponge has' is the wordplay.
'has' indicates a hidden word.
'PONG' is hidden within 'sponge'.
Can you help me to learn more?
(Other definitions for pong that I've seen before include "Reek" , "Niff" , "Bad smell" , "Stench" , "Nasty smell (informal)" .)