Very tiny rodents consuming Carl's old nuts (10)

'very tiny' is the definition.
(microscale means on a tiny scale)

'rodents consuming carl's old nuts' is the wordplay.
'rodents' becomes 'mice' (mouse is a kind of rodent).
'consuming' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'nuts' indicates anagramming the letters (nuts can mean crazy).
'carls'+'o'='carlso'
'carlso' anagrammed gives 'roscal'.
'mice' enclosing 'roscal' is 'MICROSCALE'.