Moved to a prison - having used this in food? (3-6)
I believe the answer is:
rat-poison
'used this in food?' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'moved to a prison' is the wordplay.
'moved' indicates anagramming the letters.
'to'+'a'+'prison'='toaprison'
'toaprison' is an anagram of 'RAT-POISON'.
'having' is the link.
(Other definitions for rat-poison that I've seen before include "Rodenticide" , "Vermin killer" , "Rodent killer" .)