They get high twice a day (5)
I believe the answer is:
tides
'they get high twice a day' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tides that I've seen before include "Lunar-driven ocean movements" , "Diets (anag.)" , "They ebb and flow" , "drifts" , "Those fluctuating" .)