Trick or treats possibly involving silver diadem ultimately (9)
I believe the answer is:
stratagem
'trick' is the definition.
(I've seen this in another clue)
'treats possibly involving silver diadem ultimately' is the wordplay.
'possibly' indicates anagramming the letters.
'involving' indicates putting letters inside.
'silver' becomes 'ag' (Ag is the chemical symbol for silver).
'ultimately' suggests the final letters.
The final letter of 'diadem' is 'm'.
'treats' is an anagram of 'strate'.
'strate' placed around 'ag' is 'stratage'.
'stratage'+'m'='STRATAGEM'
'or' is the link.
(Other definitions for stratagem that I've seen before include "Crafty manoeuvre" , "Cunning plot" , "Ploy - start game (anag)" , "A cunning plan or scheme" , "Device" .)