Water-bird, lake one, tucking into nut (7)
I believe the answer is:
pelican
'water-bird' is the definition.
I can't tell whether this defines the answer.
'lake one tucking into nut' is the wordplay.
'lake' becomes 'l'.
'one' becomes 'i' (roman numeral).
'tucking into' is an insertion indicator.
'nut' becomes 'pecan' (pecan is a kind of nut).
'l'+'i'='li'
'li' put inside 'pecan' is 'PELICAN'.
Can you help me to learn more?
(Other definitions for pelican that I've seen before include "Water bird with throat pouch for fish" , "one with huge bill" , "Seabird having a large bill with a pouch for fish" , "Tropical bird" , "Aquatic bird - type of road crossing" .)