Cover fastens in case of effervescence (7)
I believe the answer is:
eclipse
'cover' is the definition.
(thesaurus)
'fastens in case of effervescence' is the wordplay.
'fastens' becomes 'clips' (I can't justify this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'case of' says to hollow out the word (remove centre letters) (like emptying a case).
'effervescence' with its centre removed is 'ee'.
'clips' placed inside 'ee' is 'ECLIPSE'.
Can you help me to learn more?
(Other definitions for eclipse that I've seen before include "Lunar/solar phenomenon" , "Dominate - put in the shade" , "Decline" , "Hiding of one body by another" , "Total - - of the sun" .)