English post boxes few and far between (6)
I believe the answer is:
sparse
'few and far between' is the definition.
(I've seen this before)
'english post boxes' is the wordplay.
'english' becomes 'E' (abbreviation).
'post' is a charade indicator (letters next to each other) (post- can mean after).
'boxes' becomes 'spars' (sparring is a kind of boxing).
'e' put after 'spars' is 'SPARSE'.
(Other definitions for sparse that I've seen before include "not packed together" , "Meagre; scarce" , "Thinly dotted about" , "Rare" , "Meagre, thin on the ground" .)