This will give the precise time to discontinue guard duty (9)
I believe the answer is:
stopwatch
'this will give the precise time' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'discontinue guard duty' is the wordplay.
I cannot quite understand how this works, but
'discontinue' could be 'stop' (synonyms) and 'stop' is found within the answer.
'guard' could be 'watch' (watch is a kind of guard) and 'watch' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for stopwatch that I've seen before include "Timing device, for sports perhaps" , "Timepiece for exact timing" , "Timepiece for recording speed of racers" , "instrument" , "Timepiece used in racing" .)