Some show a very clear undulation (4)
I believe the answer is:
wave
'undulation' is the definition.
(I've seen this before)
'some show a very clear' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'WAVE' is hidden in the letters of 'show a very clear'.
(Other definitions for wave that I've seen before include "Drive forward" , "White Horse" , "breaker, for example" , "Roller; a gesture" , "Hand gesture - ridge of moving water" .)