Prolong a term at His Majesty's pleasure (7)
I believe the answer is:
stretch
'prolong' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for stretch that I've seen before include "Time inside" , "Bit of exercise" , "Prison sentence (colloquial)" , "extended" , "Period inside" .)