Are only involved at a prior stage (5,2)
I believe the answer is:
early on
'at a prior stage' is the definition.
I can't judge whether this defines the answer.
'are only involved' is the wordplay.
'involved' indicates an anagram (involved can mean confusing or complex).
'are'+'only'='areonly'
'areonly' with letters rearranged gives 'EARLY ON'.
Can you help me to learn more?
(Other definitions for early on that I've seen before include "while still young?" , "not long after starting" , "Quite soon in the proceedings" , "Towards the beginning" , "before much time passes" .)