Hi Fop, Hi Warden,
18d - 3 letter abbr. for "arrived" + single letter for "a little isthmus" in(side) "Med" - definition is "took the plunge".
25a - Is very clever - the second word in the answer is an indicator of what to do with "the rags".
13d - If you replace the last 3 letter word of the answer where they occur in "hit a toff" with the first 3 letter word you get a phrase meaning "click".