It takes a measure of time to cancel the guard (9)
I believe the answer is:
stopwatch
'it takes a measure of time' is the definition.
The definition suggests a singular noun which matches the answer.
'cancel the guard' is the wordplay.
'cancel' becomes 'stop' (I can't explain this - if you can you should give a lot more credence to this answer).
'the guard' becomes 'watch' (watch is a kind of guard).
'stop'+'watch'='STOPWATCH'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for stopwatch that I've seen before include "instrument" , "Timing device, for sports perhaps" , "Timepiece for exact timing" , "Timer for races, etc" , "Timepiece for timing, a race perhaps" .)