Tranquilize (6)
I believe the answer is:
sedate
'tranquilize' is the definition.
(I know that tranquilize can be written as sedate)
This is the entire clue.
(Other definitions for sedate that I've seen before include "Administer a calming drug" , "Composed and staid" , "Tranquilise" , "Not very exciting" , "Staid - teased (anag)" .)