Substance ultimately covered by jam? (5)
I believe the answer is:
cream
'jam?' is the definition.
(thesaurus)
'substance ultimately covered by' is the wordplay.
'substance' becomes 'C' (C is an example).
'ultimately covered' becomes 'ream' (I can't justify this - if you can you should believe this answer much more).
'by' means one lot of letters go next to another.
'c'+'ream'='CREAM'
(Other definitions for cream that I've seen before include "Top of milk or society" , "Pick - fatty part of milk" , "Very best; pale yellow" , "Best - substitute for shaving soap" , "best people" .)