In extra small, few and far between (6)
I believe the answer is:
sparse
'few and far between' is the definition.
(I've seen this in another clue)
'in extra small' is the wordplay.
'in' indicates putting letters inside.
'extra' becomes 'spare' (both can mean left over).
'small' becomes 's' (abbreviation - e.g. clothes size).
'spare' going around 's' is 'SPARSE'.
(Other definitions for sparse that I've seen before include "Thin on the ground, scarce" , "Scarce like thinning hair" , "Thinly spread" , "Thinly scattered, not dense" , "Thinly dispersed" .)