Stefan, every so often, waves (3)
I believe the answer is:
sea
'waves' is the definition.
(I've seen this in other clues)
'stefan every so often' is the wordplay.
'every so often' indicates alternate letters.
The alternate letters of 'stefan' are 'SEA'.
(Other definitions for sea that I've seen before include "70% of the earth's surface" , "Eg, Aral, Ross, Irish" , "Adriatic, for example" , "The drink" , "E.g. Baltic" .)