A second test, with time advancing, of main communication channel? (8)
I believe the answer is:
arterial
'of main communication channel?' is the definition.
(as in 'arteries' in transport and communication)
'a second test with time advancing' is the wordplay.
I cannot quite see how this works, but
'a' is within the answer.
'time' could be 't' (abbreviation) and 't' is found within the answer.
'second test' could be 'e' (2nd letter of 'test') and 'e' is found in the answer.
This explanation may well be incorrect...
(Other definitions for arterial that I've seen before include "along major roads" , "Relating to the major blood vessels" , "Like main way" , "like the bloody tube!" , "A trailer (anag.)" .)