Permitted to escape deadly sentient computer (3)
I believe the answer is:
hal
'sentient computer' is the definition.
The definition suggests a singular noun which matches the answer.
'permitted to escape deadly' is the wordplay.
'permitted' becomes 'let' (to let something happen is to permit it).
'to escape' is a deletion indicator.
'deadly' becomes 'lethal' (synonyms).
'lethal' with 'let' removed is 'HAL'.
Can you help me to learn more?
(Other definitions for hal that I've seen before include "Harry, in short" , "future Henry V" , "Odyssean character" , "Shakespeare's Prince_with Falstaff" , "Henry, in short" .)