As far as I can tell, this is a measure used mostly in the UK, and refers to dessert spoon. At one point, this was approximately 1/2 tablespoon, but has been standardized to refer to a 10 ml measure, while a UK tablespoon is 15 ml. In the US, a tablespoon is 14.79 ml, so it makes a difference what your point of reference is. So, assuming you are in the UK area:
1 Dssp = 10 ml
My medicine cup has markings for 5 different tablespoon measurements and 9 different milliliter measurements. 1 DSSP still equals 7.5 ml (or 1/2 tablespoon) on my modern, translucent plastic cup.
DSSP stands for "dessertspoon". It is a bit smaller than the tablespoon and a bit larger than the teaspoon.