Harmful substance one gets in a spoon, possibly (6)
I believe the answer is:
poison
'harmful substance' is the definition.
(I've seen this before)
'one gets in a spoon possibly' is the wordplay.
'one' becomes 'i' (Roman numeral).
'gets in' is an insertion indicator.
'possibly' indicates anagramming the letters.
'spoon' with letters rearranged gives 'poson'.
'i' inserted into 'poson' is 'POISON'.
(Other definitions for poison that I've seen before include "Eg, toxin" , "Venom, toxin" , "Deadly substance" , "[ADDER] 's defence" , "Meat for someone else" .)