Regular structure (7)
I believe the answer is:
lattice
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">regular structure</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for lattice that I've seen before include "Framework in diagonal pattern" , "Framework of crossed bars as in windows" , "Regular atomic structure" , "Network of crossed bars" , "Trellis: maybe" .)