Confirm answer in very remarkable detail (8)
I believe the answer is:
validate
'confirm' is the definition.
(validating is a kind of confirming)
'answer in very remarkable detail' is the wordplay.
'answer' becomes 'a' (abbreviation eg in Q and A).
'in' means one lot of letters goes inside another.
'very' becomes 'v' (abbreviation).
'remarkable' is an anagram indicator.
'detail' is an anagram of 'lidate'.
'v'+'lidate'='vlidate'
'a' going within 'vlidate' is 'VALIDATE'.
(Other definitions for validate that I've seen before include "Long poem" , "Verify" , "Confirm or authenticate" , "Declare legally acceptable" , "stand by" .)