23 down
Occasional
infusion
is
required
on
that
basis
(2,2)
Occasional infusion is required on that basis (2,2)
hints
0
checks
0
hints
The definition is at the end of the clue.
How the clue works
The definition is on that basis, which fits neatly. Now for the wordplay — take the occasional, meaning odd-positioned, letters of infusion: the 1st, 3rd, 5th, and 7th letters give you I, F, S, O.