Perhaps seal a tiny creature in a hard covering (4)
I believe the answer is:
pupa
'tiny creature in a hard covering' is the definition.
(insect in a hard cover)
'perhaps seal a' is the wordplay.
'perhaps seal' becomes 'pup' (pup can mean a young seal).
'pup'+'a'='PUPA'
(Other definitions for pupa that I've seen before include "Stage of an insect's development" , "A chrysalis as of butterfly" , "Phase of insect life" , "One is growing" , "creepy-crawly" .)