Speak of incarcerating one thief (6)
I believe the answer is:
pirate
'thief' is the definition.
(pirates are thieves)
'speak of incarcerating one' is the wordplay.
I cannot really see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
'speak' could be 'prate' (prating is a kind of speaking) and 'prate' is found within the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for pirate that I've seen before include "Penzance marauder ?" , "sailor" , "Cornishman maybe" , "Plagiarise" , "He sailed under the Jolly Roger" .)