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' indicates putting letters inside.
'case of' means to remove the middle letters (outsides of).
'effervescence' with its middle taken out is 'ee'.
'clips' inserted 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" .)