Put a T in "soothe" (7)
I believe the answer is:
placate
'soothe' is the definition.
(I know that placate is a more specific form of the action calm)
'put a t in' is the wordplay.
'put' becomes 'place' (place can mean to put something somewhere).
'in' is an insertion indicator.
'a'+'t'='at'
'place' placed around 'at' is 'PLACATE'.
(Other definitions for placate that I've seen before include "Calm, pacify, appease" , "Pale cat could pacify" , "Make calm, conciliate" , "Mollify, soothe anger" , "Mollify, appease" .)