Guardian setter perhaps under close observation (8)
I believe the answer is:
watchdog
'guardian' is the definition.
(I know that watchdog is a type of guardian)
'setter perhaps under close observation' is the wordplay.
'setter perhaps' becomes 'dog' (setter is a type of dog).
'under' means one lot of letters go next to another (in a down clue, some letters go below others).
'close observation' becomes 'watch' (I can't explain this - if you can you should give a lot more credence to this answer).
'dog' after 'watch' is 'WATCHDOG'.
Can you help me to learn more?
(Other definitions for watchdog that I've seen before include "Organisation guarding against inefficient or illegal practices" , "Person or canine acting as guardian" , "TV show" , "TV consumer programme" , "protector" .)