Half of them notice hold-up and become frozen (4,2)

'half' is the definition.
The answer and definition are different parts of speech. However, adjectives and past participle verbs can sometimes define each other.

'them notice hold-up and become frozen' is the wordplay.
I cannot really understand how this works, but
'up' is found in the answer.
'frozen' could be 'iced' (I've seen this in another clue) and 'iced' is located in the answer.
No letters remain.
This explanation may well be incorrect...

'of' is the link.

Can you help me to learn more?