Sandwich filler (4)
I believe the answer is:
tuna
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sandwich filler</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 tuna that I've seen before include "Marine food fish" , "Widely-eaten fish" , "Sea creature" , "Yellow-finned fish" , "Marine food for your strange aunt" .)