Visit informally (4,2)
I believe the answer is:
drop in
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">visit informally</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for drop in that I've seen before include "Pay a casual visit - by parachute" , "Briefly visit" , "Visit informally" , "home visit" , "Pay a casual visit unannounced" .)