Cover contents of glider (3)
I believe the answer is:
lid
'cover' is the definition.
(lid is a kind of cover)
'contents of glider' is the wordplay.
'contents of' indicates the answer is hidden within the clue.
'LID' can be found hidden inside 'glider'.
(Other definitions for lid that I've seen before include "Top for bottle or eye protector" , "Top crust of pie" , "Cover as for jar" , "Removable top" , "Top or cover" .)