Oddly plump cases transforming insects (5)
I believe the answer is:
pupae
'transforming insects' is the definition.
(insects which will transform into adult forms)
'oddly plump cases' is the wordplay.
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
'plump'+'cases'='plumpcases'
The alternate letters of 'plumpcases' are 'PUPAE'.
(Other definitions for pupae that I've seen before include "Chrysalises" , "Insects in the stage between larva and adult" , "Chrysalids" , "Insects in form between larvae and adults" , "Insects in the inactive stage of development" .)