A fluid we confuse with tar (5)
I believe the answer is:
water
'a fluid' is the definition.
(water is a kind of fluid)
'we confuse with tar' is the wordplay.
'confuse with' is an anagram indicator.
'we' mixed up with 'tar' can make 'WATER'.
(Other definitions for water that I've seen before include "H2O" , "What can produce solution" , "One of the four classical elements" , "Drink - type of 1 across" , "Classical element" .)