Crown calculation little bit short (6)
I believe the answer is:
summit
'crown' is the definition.
(both can mean the top of something)
'calculation little bit short' is the wordplay.
'calculation' becomes 'sum' (I have seen 'Arithmetic calculation ' mean 'sum' so perhaps 'calculation' could also mean 'sum').
'little bit' becomes 'mite' (I've seen this in another clue).
'short' means to remove the last letter.
'mite' with its last letter taken off is 'mit'.
'sum'+'mit'='SUMMIT'
(Other definitions for summit that I've seen before include "The highest peak" , "Highest point (of hill)" , "High point" , "Talks" , "Top or extreme point" .)