What could be calmer, I wonder (7)
I believe the answer is:
miracle
'wonder' is the definition.
(I've seen this before)
'what could be calmer i' is the wordplay.
'what could be' indicates an anagram.
'calmer'+'i'='calmeri'
'calmeri' with letters rearranged gives 'MIRACLE'.
(Other definitions for miracle that I've seen before include "Divinely-inspired event" , "Unexplained wonder" , "Phenomenon" , "Marvel" , "Supernatural occurrence" .)