Pleased with sandwich's first filling (8)
I believe the answer is:
contents
'filling' is the definition.
(thesaurus)
'pleased with sandwich's first' is the wordplay.
'pleased' becomes 'content' (associated in meaning).
'with' means one lot of letters go next to another.
'sandwich's first' becomes 's' (1st letter of 'sandwich').
'content'+'s'='CONTENTS'
(Other definitions for contents that I've seen before include "list of ingredients" , "All that's contained" , "Satisfies the list of chapters" , "Makes satisfied - with ingredients?" , "List of what's included" .)