They sell nameless fluid (8)
I believe the answer is:
salesmen
'they sell' is the definition.
Although both the answer and 'sell' are nouns, they are not in the same form of the noun.
'nameless fluid' is the wordplay.
'fluid' indicates anagramming the letters.
'nameless' is an anagram of 'SALESMEN'.
Can you help me to learn more?
(Other definitions for salesmen that I've seen before include "They sell goods and services" , "People engaged in trade" , "representatives" , "traders" .)