時計 is ateji, there's no logical relation between the single kanji readings and the reading of the whole word in this case. Just learn it like that. The reason is:
During Zhou dynasty, 土圭 was an apparatus used for measuring position. (The 圭 kanji used to stand for a type of tool.) When this word came to Japan, it was only used for sundials. So when in the 16th century the first actual mechanical clocks came to Japan, people started to use the word ときはかり (時計) or じめいしょう (自鳴鐘) to refer to them. The latter was the term used in China, and the former is a Japanese word.
In the end however, people apparently preferred the word とけい but it seems like they found the spelling 時計 more fitting than 土圭.
It's not pronounced とけ in this word. The individual kanji in this type of word (jukujikun) don't have an individual reading, they only have a reading as a whole. Words like 大人 (おとな) and 今日 (きょう) also follow this concept, and cannot be divided into a situation where おと is a reading for 大 or う is a reading for 日. It's merely that the reading for those pairs are the words as a whole.
Ah, I understand now. I didn't know that the pronunciation of some words did not correspond to their individual characters. Thank you for you time and explanations :)
Why is 時 is pronounced as とけ here ?
The Kun'yomi reading for 時 is とき, so why is it pronounced as とけ in this word ?