Write note around margin: 'It's Wimbledon today!' (5)
I believe the answer is:
theme
'wimbledon today' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'write note around margin' is the wordplay.
'write note' becomes 'te' (I can't justify this - if you can you should believe this answer much more).
'around' is an insertion indicator.
'margin' becomes 'hem' (I've seen this in another clue).
'te' placed around 'hem' is 'THEME'.
'it's' is the link.
Can you help me to learn more?
(Other definitions for theme that I've seen before include "tune used in Enigma Variations?" , "Unifying motif" , "Melody on which variations are constructed" , "Subject matter, motif" , "Subject (of talk, text, etc)" .)