Bird to start to undo container (6)
I believe the answer is:
toucan
'bird' is the definition.
(toucan is a kind of bird)
'to start to undo container' is the wordplay.
'start to' suggests taking the first letters.
'container' becomes 'can' (can is a kind of container**).
The first letter of 'undo' is 'u'.
'to'+'u'+'can'='TOUCAN'
(Other definitions for toucan that I've seen before include "One with huge bill" , "Coloured tropical bird with a large beak" , "Colourful bird, big bill" , "Tropical bird with large bill" , "exotic bird" .)