Vermin in crate (3)
I believe the answer is:
rat
'vermin' is the definition.
(rats are vermin)
'in crate' is the wordplay.
'in' says the answer is hidden in the clue.
'RAT' is hidden within 'crate'.
(Other definitions for rat that I've seen before include "Domestic rodent" , "Rodent; informer (colloq.)" , "Rodent pest" , "Betray like a rodent" , "Sewer rodent" .)