Trimmed the border with diamonds (5)
I believe the answer is:
edged
'trimmed' is the definition.
(thesaurus)
'the border with diamonds' is the wordplay.
'the border' becomes 'edge' ('edge' can be a synonym of 'border').
'with' means one lot of letters go next to another.
'diamonds' becomes 'd' (abbreviation in bridge say).
'edge'+'d'='EDGED'
(Other definitions for edged that I've seen before include "with the ability to cut" , "Moved sideways slowly" , "Nicked" , "sharp" , "Trimmed" .)