Common or garden finally entered in journal (8)
I believe the answer is:
ordinary
'common' is the definition.
(synonyms)
'or garden finally entered in journal' is the wordplay.
'finally' suggests the final letters.
'entered in' indicates putting letters inside.
'journal' becomes 'diary' (diary is a kind of journal).
The final letter of 'garden' is 'n'.
'n' put inside 'diary' is 'dinary'.
'or'+'dinary'='ORDINARY'
(Other definitions for ordinary that I've seen before include "Common-or-garden" , "penny-farthing years ago" , "'Usual, everyday (8)'" , "Run of the mill" , "Undistinguished" .)