When temperature is hot, bull becomes thin (5)
I believe the answer is:
sheer
'thin' is the definition.
(similar in meaning)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for sheer that I've seen before include "Absolute - precipitous" , "needing to change a course" , "Nothing other than" , "Unqualified" , "Very thin and fine" .)