Finesse diamonds in no trumps play finally (6)
I believe the answer is:
nicety
'finesse' is the definition.
(thesaurus)
'diamonds in no trumps play finally' is the wordplay.
'diamonds' becomes 'ice' (I've seen this before).
'in' means one lot of letters goes inside another.
'no trumps' becomes 'nt'.
'finally' says to take the final letters.
The last letter of 'play' is 'y'.
'ice' going into 'nt' is 'nicet'.
'nicet'+'y'='NICETY'
(Other definitions for nicety that I've seen before include "Delicacy" , "A detail of etiquette" , "Refinement" , "Fine distinction" , "Subtle difference" .)