Boxes with false bottom in short supply (6)
I believe the answer is:
sparse
'in short supply' is the definition.
(sparse can mean meagre or minimal)
'boxes with false bottom' is the wordplay.
'boxes' becomes 'spars' (sparring is a kind of boxing).
'with' says to put letters next to each other.
'bottom' suggests the final letters.
The final letter of 'false' is 'e'.
'spars'+'e'='SPARSE'
(Other definitions for sparse that I've seen before include "One's not at all thick" , "Thinly scattered, not dense" , "Thinly populated" , "Spares, being thin on the ground" , "Meagre; scarce" .)