Light tail of teal, an aquatic bird (7)
I believe the answer is:
lantern
'light' is the definition.
(lantern is a kind of light)
'tail of teal an aquatic bird' is the wordplay.
'tail of' says to take the final letters.
'aquatic bird' becomes 'tern' (tern is a kind of aquatic bird).
The final letter of 'teal' is 'l'.
'l'+'an'+'tern'='LANTERN'
(Other definitions for lantern that I've seen before include "Type of lamp" , "Light held by 26/27 Across" , "It illuminates" , "Cased lamp" , "Transparent case for holding a light" .)