Rodent within crate (3)
I believe the answer is:
rat
'rodent' is the definition.
(rat is a kind of rodent)
'within crate' is the wordplay.
'within' indicates a hidden word.
'RAT' is hidden within 'crate'.
(Other definitions for rat that I've seen before include "Rodent - deserter" , "Sewer rodent" , "Abandon" , "one who deserts the sinking ship?" , "Treacherous person" .)