25 down
Around
5/9ths
of
what
biorhythms
are
(5)
Around 5/9ths of what biorhythms are (5)
hints
0
checks
0
hints
The definition is at the beginning of the clue.
How the clue works
The definition is Around — that's your straightforward "approximately" sense. Now, biorhythms are CIRCADIAN (nine letters), and if you take 5/9ths of that word you get its first five letters, CIRCA. Note that the "5" in the clue may well be a cross-reference to 5 down rather than a plain number, but either way the fraction points you neatly at those opening letters.