Perform improperly with paste, hiding diamonds (5)
I believe the answer is:
misdo
'perform improperly' is the definition.
(I've seen this in another clue)
'paste hiding diamonds' is the wordplay.
'paste' becomes 'miso' (I've seen this before).
'hiding' indicates putting letters inside.
'diamonds' becomes 'd' (abbreviation for the card suit diamonds).
'miso' going around 'd' is 'MISDO'.
'with' acts as a link.
(Other definitions for misdo that I've seen before include "Perform wrongly" , "Perform improperly" .)