Eats sparingly
(Other definitions for picks that I've seen before include "Goes for" , "Chooses - tools for breaking rocks" , "Choices" , "tools for breaking up rock" , "More than one tool" .)
'eats sparingly' is the definition.
('diet' can be a synonym of 'eat sparingly')
This is all the clue.
(Other definitions for diets that I've seen before include "Regimens" , "Disciplined eating regimes" , "Commons" , "Tides (anag.)" , "causing stones to vanish" .)