Part of River Island's ... (4)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

IS IS = two of IS, short for island. Of course apostrophe-S doesn’t normally indicate a plural, but sometimes it’s used for plurals of abbreviations; or you could just say we should follow the usual crossword rule of ignoring punctuation. [UPDATE: As Hovis @3 points out, it’s one IS for “island” and another IS for the apostrophe-S = “is”.]  And for the sake of the surface, “River Island” is a high-street clothing brand. Name for the upper part of the River Thames, especially where it flows through Oxford, before it joins with the River Thame to become the Thames.