It is not true for one to assume a horizontal position (3)
I believe the answer is:
lie
'assume a horizontal position' is the definition.
(I've seen this before)
'it is not true for one' is the wordplay.
I cannot really see how this works, but
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is found within the answer.
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
A single letter 'l' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
'to' is the link.
Can you help me to learn more?
(Other definitions for lie that I've seen before include "Falsity" , "Prevaricate or recline" , "Stretch out" , "Story" , "Whopper" .)