Half-a-dozen beside a channel bridge (7)
I believe the answer is:
viaduct
'bridge' is the definition.
(viaduct is a kind of bridge)
'half-a-dozen beside a channel' is the wordplay.
'half-a-dozen' becomes 'vi' (I've seen this in other clues).
'beside' means one lot of letters go next to another (I've seen this in other clues).
'channel' becomes 'duct' ('duct' can be a synonym of 'channel').
'vi'+'a'+'duct'='VIADUCT'
(Other definitions for viaduct that I've seen before include "Structure carrying road or railway over a valley" , "Bridge-like structure" , "Road/rail bridge" , "Bridge to carry railway over a valley" , "Type of bridge" .)