Rodent in crate (3)
I believe the answer is:
rat
'rodent' is the definition.
(rat is a kind of rodent)
'in crate' is the wordplay.
'in' indicates the answer is hidden within the clue.
'RAT' is hidden in the letters of 'crate'.
(Other definitions for rat that I've seen before include "Grass" , "one who deserts the sinking ship?" , "one escaping doomed vessel?" , "Tell-tale" , "Desert - rodent" .)