Insulating layer perhaps seals edges of glasshouse (5)
I believe the answer is:
podge
'insulating layer' is the definition.
(podge can mean excess fat)
'seals edges of glasshouse' is the wordplay.
'seals' becomes 'pod' (pod can mean a group of marine animals).
'edges of' says to hollow out the word (remove centre letters).
'glasshouse' with its centre taken out is 'ge'.
'pod'+'ge'='PODGE'
'perhaps' acts as a link.
(Other definitions for podge that I've seen before include "Fatso" , "He needs exercise" , "One with pounds to spare" , "Fat" .)