Thin boxes, empty initially (6)
I believe the answer is:
sparse
'thin' is the definition.
(I know that thin can be written as sparse)
'boxes empty initially' is the wordplay.
'boxes' becomes 'spars' (sparring is a kind of boxing).
'initially' suggests taking the first letters.
The initial letter of 'empty' is 'e'.
'spars'+'e'='SPARSE'
(Other definitions for sparse that I've seen before include "Thinly-spread" , "Rare" , "not packed together" , "Far from lavish" , "Thinly spread" .)