Large island lake contains rudimentary yellow flower (4)
I believe the answer is:
lily
'rudimentary yellow flower' is the definition.
'lily' can be an answer for 'flower' (I've seen this before). I am not sure about the 'rudimentary yellow' bit.
'large island lake contains' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'island' becomes 'i' (geographical abbreviation).
'lake' becomes 'L' (geographical abbreviation).
'contains' becomes 'y' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'l'+'i'+'l'+'y'='LILY'
Can you help me to learn more?
(Other definitions for lily that I've seen before include "White" , "Flower; girl's name" , "Bulbous plant with trumpet-shaped flowers" , "Eg, arum, calla" , "Plant with showy pendulous flowers" .)