Relay final, remains to be seen immediately (2,1,5)
I believe the answer is:
in a flash
'immediately' is the definition.
Both the definition and answer are adverbs. Perhaps there's a link between them I don't understand?
'relay final remains to be' is the wordplay.
I cannot really see how this works, but
an anagram of 'final' is 'inafl' which is present in the answer.
'remains' could be 'ash' (I've seen this before) and 'ash' is present in the answer.
No letters remain.
This may be the basis of clue (or it may be nonsense).
'seen' is the link.
Can you help me to learn more?
(Other definitions for in a flash that I've seen before include "Almost instantly" , "Instantly, straightaway" , "Very quickly" , "How lightning appears" , "pictures may be taken" .)