Ordered cadet that is controlling tank to turn off (10)

'turn off' is the definition.
(to turn a device off is to deactivate it)

'ordered cadet that is controlling tank' is the wordplay.
'ordered' indicates anagramming the letters.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'controlling' indicates putting letters inside.
'tank' becomes 'vat' (type of container).
'ie' placed around 'vat' is 'ivate'.
'cadet' is an anagram of 'deact'.
'deact'+'ivate'='DEACTIVATE'

'to' acts as a link.