While travelling (2,5)
I believe the answer is:
en route
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">while travelling</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 en route that I've seen before include "'On the way (2,5)'" , "While travelling" , "on the road" , "Along the way" , "March" .)