Fixed rates as well as variable, not the real thing (6)
I believe the answer is:
ersatz
'not the real thing' is the definition.
(I've seen this in another clue)
'fixed rates as well as variable' is the wordplay.
'fixed' indicates anagramming the letters.
'as well as' says to put letters next to each other.
'variable' becomes 'z' (algebra).
'rates' with letters rearranged gives 'ersat'.
'ersat'+'z'='ERSATZ'
(Other definitions for ersatz that I've seen before include "False" , "Cheap substitute or imitation" , "Inferior substitute" , "Artificial or synthetic, from German" , "Spurious" .)