Cleaner finding nasty smell in surface externally (6)
I believe the answer is:
sponge
'cleaner' is the definition.
'nasty smell in surface externally' is the wordplay.
'nasty smell' becomes 'pong' (I've seen this before).
'in' means one lot of letters goes inside another.
'externally' says to hollow out the word (remove centre letters) (only the external letters are used).
'surface' with its middle taken out is 'se'.
'pong' going inside 'se' is 'SPONGE'.
'finding' acts as a link.
(Other definitions for sponge that I've seen before include "Aquatic invertebrate with a soft porous body" , "Sea animal often found in bath" , "drunkard or > hanger-on" , "Wipe" , "Ask for money" .)