Box, additional, but not full (4)
I believe the answer is:
spar
'box' is the definition.
(spar can mean to box)
'additional but not full' is the wordplay.
'additional' becomes 'spare' (I've seen this before).
'but not full' means to remove the last letter.
'spare' with its final letter taken off is 'SPAR'.
(Other definitions for spar that I've seen before include "Mineral; box; wing beam" , "supporting member" , "Length of wood" , "microcrystalline mineral?" , "Close friend" .)