Poisonous snake in back of trunk, one kept by scoundrel (5)
I believe the answer is:
krait
'poisonous snake' is the definition.
(I've seen this before)
'back of trunk one kept by scoundrel' is the wordplay.
'back of' says to take the final letters.
'one' becomes 'I' (Roman numeral).
'kept by' indicates putting letters inside.
'scoundrel' becomes 'rat' (I've seen this in other clues).
The last letter of 'trunk' is 'k'.
'i' going into 'rat' is 'rait'.
'k'+'rait'='KRAIT'
'in' acts as a link.
(Other definitions for krait that I've seen before include "Out East, snake" , "Venomous snake of southeast Asia" , "Venomous Asian snake" , "Poisonous snake" , "Deadly snake" .)