Calm provided in fast-moving surroundings (6)
I believe the answer is:
pacify
'calm' is the definition.
(pacifying is a kind of calming)
'provided in fast-moving surroundings' is the wordplay.
'provided in' becomes 'if' (I am not sure about the 'in' bit.).
'fast-moving' becomes 'pacy'.
'surroundings' indicates putting letters inside.
'if' put inside 'pacy' is 'PACIFY'.
(Other definitions for pacify that I've seen before include "Soothe - appease" , "Appease - subdue" , "Make calmer" , "Quieten" , "Placate, calm" .)