Fluid taken in by hippopotamus, say — one that's shy? (10)
I believe the answer is:
wallflower
'one that's shy?' is the definition.
(wallflower can mean a shy person)
'fluid taken in by hippopotamus say' is the wordplay.
'fluid' becomes 'fl' (eg in fl oz, fluid ounce).
'taken in by' means one lot of letters goes inside another.
'hippopotamus say' becomes 'wallower' (an animal which wallows).
'fl' inserted inside 'wallower' is 'WALLFLOWER'.
(Other definitions for wallflower that I've seen before include "Dance spectator; plant" , "attracts no attention" , "one's sidelined" , "One lacking a partner - garden plant" , "Person waiting in vain to be asked to dance" .)