Crossword Genius

I have no idea why insect circles new mast (5,2)

Ross

I believe the answer is:

beats me

'i have no idea why' is the definition.

'insect circles new mast' is the wordplay.
'insect' becomes 'bee' (bee is a type of insect).
'circles' is an insertion indicator.
'new' indicates anagramming the letters.
'mast' anagrammed gives 'atsm'.
'bee' enclosing 'atsm' is 'BEATS ME'.

I've seen this clue in The Independent.
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