Major celebrity of late briefly wearing policeman’s medal (12)
I believe the answer is:
superstardom
'major celebrity' is the definition.
'policeman's medal' is the wordplay.
'policeman' becomes 'super' (short for superintendant).
'medal' becomes 'stardom' (I can't explain this - if you can you should give a lot more credence to this answer).
'super'+'stardom'='SUPERSTARDOM'
'of late briefly wearing' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
(Other definitions for superstardom that I've seen before include "Realm of A-list celebrity?" , "Immense fame" , "Position of great fame" .)