Calm a steed that's wild (6)
I believe the answer is:
sedate
'calm' is the definition.
(I know that calm can be written as sedate)
'a steed that's wild' is the wordplay.
'that's wild' is an anagram indicator.
'a'+'steed'='asteed'
'asteed' anagrammed gives 'SEDATE'.
(Other definitions for sedate that I've seen before include "boring" , "Unexciting" , "Calm, unhurried" , "Give calming drug to" , "Tranquilise" .)