Mixture of wee, gas and effluent (6)
I believe the answer is:
sewage
'effluent' is the definition.
(I've seen this before)
'mixture of wee gas' is the wordplay.
'mixture of' is an anagram indicator (I've seen this in other clues).
'wee'+'gas'='weegas'
'weegas' is an anagram of 'SEWAGE'.
'and' acts as a link.
(Other definitions for sewage that I've seen before include "Refuse for drains" , "Product of toilet" , "Refuse carried off through drainage system" , "Effluence" , "Liquid waste" .)