4 down
One on prickly cacti I found on surface of desert – that’s sharp! (6,4)
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How the clue works
The definition is that’s sharp, as in a sharp-tasting acid. Start with ACE for 'one', then add an anagram of CACTI, I and D (the first letter, or 'surface', of Desert), with 'prickly' as the anagram indicator. The jumbled letters make TICACID, which gives ACETIC ACID when attached to ACE.