What’s immediate sensation mixed with tuna (13)
I believe the answer is:
instantaneous
'what's immediate' is the definition.
(similar in meaning)
'sensation mixed with tuna' is the wordplay.
'mixed with' indicates anagramming the letters.
'sensation' anagrammed with 'tuna' makes 'INSTANTANEOUS'.
(Other definitions for instantaneous that I've seen before include "Occurring in a moment" , "Momentary" , "Direct" , "On-the-spot" , "Done in a moment" .)