The homework question that I just posted at 7:03pm is for a kindengarden.

Answer 1

That still doesn't tell me what is meant by "clusters of words."

I guess start by listing all the words you know that start with FL or that have FL in them somewhere.

Did you even go to onelook.com and look up FL* words?
http://www.onelook.com/?w=fl*&first=101

Answer 2

Be sure you list only the words that the kindergartener knows!!

Answer 3

Another way to think of this is to list words in "families" --

fly
flee
flight
flew
flown

flat
flatter
flattest

and so forth