Large bird in lake, one tucking into nut (7)
I believe the answer is:
pelican
'large bird' is the definition.
(I've seen this before)
'lake one tucking into nut' is the wordplay.
'lake' becomes 'L' (geographical abbreviation).
'one' becomes 'i' (Roman numeral).
'tucking into' indicates putting letters inside.
'nut' becomes 'pecan' (type of nut).
'l'+'i'='li'
'li' placed inside 'pecan' is 'PELICAN'.
'in' acts as a link.
(Other definitions for pelican that I've seen before include "Bird's" , "Large aquatic bird" , "The large bird is in place" , "one with huge bill" , "Bird with large pouched beak" .)