Working in atrial fibrillation is sensible (8)
I believe the answer is:
rational
'sensible' is the definition.
(associated in meaning)
'working in atrial fibrillation' is the wordplay.
'working' becomes 'on' (on can mean working or activated).
'in' means one lot of letters goes inside another.
'fibrillation' is an anagram indicator (letters quiver or fibrillate).
'atrial' anagrammed gives 'ratial'.
'on' inserted inside 'ratial' is 'RATIONAL'.
'is' acts as a link.
(Other definitions for rational that I've seen before include "Sane, based on logic" , "Sane and sensible" , "Capable of thought" , "Based on reason or logic" , "Thinking - not a liar (anag)" .)