Crustacean is beginning to outgrow shell (6)
I believe the answer is:
isopod
'crustacean' is the definition.
(I know that isopod is a type of malacostracan crustacean)
'is beginning to outgrow shell' is the wordplay.
'beginning to' indicates taking the first letters.
'shell' becomes 'pod' (synonyms).
The first letter of 'outgrow' is 'o'.
'is'+'o'+'pod'='ISOPOD'
(Other definitions for isopod that I've seen before include "Crawler" , "This crusty creature" , "Member of an order of crustaceans" , "small crustacean" , "Crustacean such as a woodlouse" .)