Check provided in extreme case (6)
I believe the answer is:
verify
'check' is the definition.
(to verify is to check something is true)
'provided in extreme case' is the wordplay.
'provided' becomes 'if' (eg 'provided it's sunny' means 'if it's sunny').
'in' means one lot of letters goes inside another.
'extreme case' becomes 'very' (I can't justify this - if you can you should believe this answer much more).
'if' going within 'very' is 'VERIFY'.
Can you help me to learn more?
(Other definitions for verify that I've seen before include "Support" , "Authenticate" , "Check (facts)" , "Confirm the truth of" , "Establish truth or accuracy of" .)