Left the top off timepiece - it's crucial for getting into the internal housing
I believe the answer is:
latchkey
'the internal housing' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'left the top off timepiece it's crucial for getting' is the wordplay.
I cannot really understand how this works, but
'left' could be 'l' (common abbreviation) and 'l' is found in the answer.
'crucial' could be 'key' (I've seen this before) and 'key' is found within the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
This may be the basis of clue (or it may be nonsense).
'into' is the link.
Can you help me to learn more?
(Other definitions for latchkey that I've seen before include "Common device to open outer door" , "Sort of self-reliant child" , "Means of entry to (parentless?) house" , "A supposed freedom from domestic restrictions" , "Describes a child who returns to an empty home" .)