Splendid wave (5)
I believe the answer is:
swell
This clue is a double definition.
'splendid' is the first definition.
(I've seen this before)
'wave' is the second definition.
(swell is a kind of wave)
(Other definitions for swell that I've seen before include "Enlarge; excellent" , "Rise into waves" , "Expand abnormally" , "'Increase in size, number etc (5)'" , "Burgeon" .)