Bridge bidding system (4)
I believe the answer is:
acol
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">bridge bidding system</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 acol that I've seen before include "Bridge convention" , "Way to bid" , "Bidding system in bridge" , "often seen in bridge" .)