Object concealing trap first off in horror film (3,7)

'horror film' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'object concealing trap first off in' is the wordplay.
I cannot really see how this works, but
'object' could be 'th' (Th is an example) and 'th' is present in the answer.
'first' could be 'i' (I in Roman numerals eg Elizabeth I) and 'i' is located in the answer.
'in' is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?