Passer-by fits in short supply (6)
I believe the answer is:
sparse
'in short supply' is the definition.
(sparse can mean meagre or minimal)
'passer-by fits' is the wordplay.
'passer' anagrams to 'SPARSE'.
However, I'm unsure how this is indicated.
Can you help me to learn more?
(Other definitions for sparse that I've seen before include "Rare" , "Far from lavish" , "Meagre, scanty" , "Not much" , "One's not at all thick" .)