Supplementary membership fees cover one journal (10)
I believe the answer is:
subsidiary
'supplementary' is the definition.
(I know that supplementary can be written as subsidiary)
'membership fees cover one journal' is the wordplay.
'membership fees' becomes 'subs' (fees paid to a club or organisation).
'cover one' becomes 'i' (I can't justify this - if you can you should believe this answer much more).
'journal' becomes 'diary' (synonyms).
'subs'+'i'+'diary'='SUBSIDIARY'
Can you help me to learn more?
(Other definitions for subsidiary that I've seen before include "Contributory" , "company" , "Less important" , "of lesser importance" , "Minor" .)