Restraint kept in crate there (6)
I believe the answer is:
tether
'restraint' is the definition.
(I know that tether is a type of restraint)
'in crate there' is the wordplay.
'in' indicates the answer is hidden within the clue.
'TETHER' is hidden within 'crate there'.
'kept' acts as a link.
(Other definitions for tether that I've seen before include "Rope restricting animal's movement" , "Restraint for animal" , "Rope for hobbling animal" , "Tie up (horse)" , "Tie animal to prevent from straying" .)