A wave that flattens out (6)
I believe the answer is:
roller
'a wave that flattens out' is the definition.
'roller' can be an answer for 'a wave' (roller is a kind of wave). I'm not certain of the rest of the definition.
'wave that flattens out' is the wordplay.
'wave' becomes 'roll' (roll is a kind of wave).
'that flattens out' becomes 'er' (I can't justify this - if you can you should give a lot more credence to this answer).
'roll'+'er'='ROLLER'
Can you help me to learn more?
(Other definitions for roller that I've seen before include "Long heavy sea wave" , "Long heavy wave approaching the shore" , "Painting tool" , "Wave; castor; cylinder" , "road maintenance vehicle" .)