Upon reflection, an aid to location (5)
I believe the answer is:
sonar
'location' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'upon reflection an aid' is the wordplay.
I cannot really see how this works, but
'upon' could be 'on' (I've seen this in another clue) and 'on' is located in the answer.
'an' could be 'a' and 'a' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'to' is the link.
Can you help me to learn more?
(Other definitions for sonar that I've seen before include "Means of detecting" , "Underwater detection system" , "Echo-sounding equipment" , "Echo-sounding device" , "Device for detecting objects under water with sound waves" .)