Trembles when royal escapes shark with enormous head and tail (6)
I believe the answer is:
shakes
'trembles' is the definition.
('shake' can be a synonym of 'tremble')
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for shakes that I've seen before include "Judders" , "jars" , "Quivers" , "Frothy drink of milk and flavouring" , "Moves quickly up and down, say milk drink" .)