Actors relieved initially, finding system (5)
I believe the answer is:
radar
'system' is the definition.
(I have seen 'System of detection' mean 'radar' so perhaps 'system' could also mean 'radar')
'actors relieved initially' is the wordplay.
'actors' becomes 'rada' (I can't explain this - if you can you should give a lot more credence to this answer).
'initially' suggests taking the first letters.
The first letter of 'relieved' is 'r'.
'rada'+'r'='RADAR'
'finding' is the link.
Can you help me to learn more?
(Other definitions for radar that I've seen before include "one sends out signals" , "Method of detection" , "What shows range" , "Palindromic tracking system" , "Range and direction finding equipment" .)