Valley bridge (7)
I believe the answer is:
viaduct
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">valley bridge</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for viaduct that I've seen before include "Type of bridge" , "Bridge across a valley" , "Long arched structure" , "Structure carrying road or railway over a valley" , "Bridge-like structure" .)