Light tail of razorbill, an aquatic bird (7)
I believe the answer is:
lantern
'light' is the definition.
(lantern is a type of light)
'tail of razorbill an aquatic bird' is the wordplay.
'tail of' suggests the final letters.
'aquatic bird' becomes 'tern' (tern is a kind of aquatic bird).
The last letter of 'razorbill' is 'l'.
'l'+'an'+'tern'='LANTERN'
(Other definitions for lantern that I've seen before include "Encased light" , "Lamp with protective transparent casing" , "Light in a protective casing" , "Type of lamp" , "Transparent case for candle" .)