It makes me sick when one's taken in by spoon-bending (6)
I believe the answer is:
poison
'it makes me sick' is the definition.
'one's taken in by spoon-bending' is the wordplay.
'one' becomes 'i' (Roman numeral).
'taken in by' is an insertion indicator.
'bending' indicates anagramming the letters.
'spoon' anagrammed gives 'poson'.
'i' going within 'poson' is 'POISON'.
'when' is the link.
(Other definitions for poison that I've seen before include "Eg, strychnine" , "Damaging stuff - drink?" , "Deadly substance" , "Substance that damages tissues or causes death" , "[ADDER] 's defence" .)