Calm, I start to feel, during fast (6)
I believe the answer is:
pacify
'calm' is the definition.
(I know that pacify is a kind of calm)
'i start to feel during fast' is the wordplay.
'start to' says to take the initial letters.
'during' indicates putting letters inside.
'fast' becomes 'pacy' (I've seen this before).
The initial letter of 'feel' is 'f'.
'i'+'f'='if'
'if' put inside 'pacy' is 'PACIFY'.
(Other definitions for pacify that I've seen before include "Quieten" , "Appease - subdue" , "Soothe - appease" , "Make calm, soothe" , "Make calmer" .)