The day before, new head's calm (8)
I believe the answer is:
evenness
'calm' is the definition.
(I have seen 'great calm' mean 'evenness' so perhaps 'calm' could also mean 'evenness')
'the day before new head's' is the wordplay.
'the day before' becomes 'eve' (eve is the night before something).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'head' becomes 'ness' (type of geographical point).
'eve'+'n'+'ness'='EVENNESS'
(Other definitions for evenness that I've seen before include "Smooth quality" , "consistency" , "Levelness" , "Quality of the uniform" , "Uniformity, smoothness" .)