Cook rhubarb with fish? That's revolutionary (6)
I believe the answer is:
doctor
'cook' is the definition.
(I've seen this before)
'rhubarb with fish? that's revolutionary' is the wordplay.
'rhubarb' becomes 'rot' (both can mean nonsense or false information).
'with' says to put letters next to each other.
'fish?' becomes 'cod' (type of fish).
'that's revolutionary' shows that the letters should be reversed in order (letters revolve around).
'rot'+'cod'='rotcod'
'rotcod' backwards is 'DOCTOR'.
(Other definitions for doctor that I've seen before include "Falsify; quack" , "Tamper with - falsify - one of the 14 films" , "Spike?" , "19 [WHO] , say" , "Medic or holder of highest university degree" .)