Pick out one very good behind (7)
I believe the answer is:
isolate
'pick out' is the definition.
(I've seen this before)
'one very good behind' is the wordplay.
I cannot really understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is present in the answer.
'very' could be 'so' (both can mean 'extremely') and 'so' is present in the answer.
'behind' could be 'late' (as in behind schedule) and 'late' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for isolate that I've seen before include "Place apart, away from others" , "Cut off from all contact" , "Keep apart" , "divorce" , "Set apart from others" .)