Principal pipes (5)
I believe the answer is:
mains
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">principal pipes</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for mains that I've seen before include "Water, gas or electricity supply" , "Distribution network" , "They're located in cockpit" , "power supply" , "Source of electricity or gas" .)