Thin box with square ends (6)
I believe the answer is:
sparse
'thin' is the definition.
(I know that thin can be written as sparse)
'box with square ends' is the wordplay.
'box' becomes 'spar' (spar can mean to box).
'with' says to put letters next to each other.
'ends' suggests removing the centre.
'square' with its centre taken out is 'se'.
'spar'+'se'='SPARSE'
(Other definitions for sparse that I've seen before include "One's not at all thick" , "Thinly distributed" , "Thinly-spread" , "Meagre, scanty" , "Meagre; scarce" .)