Reform as term in Dartmoor detains ringleader (10)
I believe the answer is:
mastermind
'ringleader' is the definition.
(I've seen this before)
'reform as term in dartmoor detains' is the wordplay.
'detains' indicates the answer is hidden within the clue (the answer is detained by the clue).
'MASTERMIND' is hidden within 'reform as term in dartmoor'.
(Other definitions for mastermind that I've seen before include "TV knowledge contest" , "Ringleader" , "Person conceiving a project - TV programme" , "brainbox" , "Brainy TV quiz" .)