Channel carrying hydrogen across its length (7)
I believe the answer is:
through
'across its length' is the definition.
(as in going across or through something)
'channel carrying hydrogen' is the wordplay.
'channel' becomes 'trough' (trough is a kind of channel).
'carrying' is an insertion indicator.
'hydrogen' becomes 'h' (H is the chemical symbol for hydrogen).
'trough' placed around 'h' is 'THROUGH'.
(Other definitions for through that I've seen before include "having arrived at next stage" , "By means of, via" , "made it to the next stage" , "By way of - exhausted" , "Finished; by way of" .)