Sadness is beginning to moderate in time (6)
I believe the answer is:
dismay
'sadness' is the definition.
The definition and answer can be both feelings as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'is beginning to moderate in time' is the wordplay.
'beginning to' indicates taking the first letters.
'in' is an insertion indicator.
'time' becomes 'day' (day is a kind of time).
The initial letter of 'moderate' is 'm'.
'is'+'m'='ism'
'ism' inserted within 'day' is 'DISMAY'.
Can you help me to learn more?
(Other definitions for dismay that I've seen before include "Consternation, distress" , "Affright" , "Sadden" , "Appal" , "upset" .)