Yes, a similar clue "in music a single frequency tone" was asked two weeks ago . There was no letter count given there either and the suggestions given were "pure tone" and "partial".
Partial seem to be the answer here but- just a tip- it's always a good idea to give the full clue and letter count plus details of any known letters
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Do you know the answer?
Daily Telegraph Prize Crossword 54 Saturday April 29Th 2017