They're eaten up in the stratosphere (5)
I believe the answer is:
tarts
'they're eaten up' is the definition.
The definition suggests an adverb but the answer is not.
'the stratosphere' is the wordplay.
'TARTS' is hidden backwards within the letters.
But, I'm not sure how the hidden word is indicated.
'in' is the link.
Can you help me to learn more?
(Other definitions for tarts that I've seen before include "Pastry cases with fillings" , "Broads" , "Flans" , "Small open pies" , "Hookers" .)