/k/ directly before another plosive, such as /t/, is not audibibly released. In normal conversation, you don't really hear the /k/ at all. In 'clock', the /k/ is released laterally, over the sides of the tongue.
The result is that we produce and hear two different versions (allophones) of the phoneme /k/. Both of these allophones are different again from the /k/ in, for example 'cat', in which it comes before a vowel and is fully released.
Incidentally, 'electricity is usually transcribed in Br E as /ɪlekˈtrɪsɪti/