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
1 Dssp = 10 ml