New diets bring their ups and downs (5)
I believe the answer is:
tides
'their ups and downs' is the definition.
I can't judge whether this definition defines the answer.
'new diets' is the wordplay.
'new' indicates an anagram.
'diets' with letters rearranged gives 'TIDES'.
'bring' is the link.
Can you help me to learn more?
(Other definitions for tides that I've seen before include "Those fluctuating" , "Sited (anag.)" , "They ebb and flow" , "Lunar-driven sea movements (5)" , "Is Ted going in and out?" .)