Thin box with square ends (6)
I believe the answer is:
sparse
'thin' is the definition.
(synonyms)
'box with square ends' is the wordplay.
'box' becomes 'spar' (spar can mean to box).
'with' is a charade indicator (letters next to each other).
'ends' suggests removing the centre.
'square' with its middle removed 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" .)