Son has no trousers (6)
I believe the answer is:
slacks
'trousers' is the definition.
(type of trousers)
'son has no' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'has no' becomes 'lacks'.
's'+'lacks'='SLACKS'
(Other definitions for slacks that I've seen before include "casual attire" , "bags" , "takes it easy" , "Pants for casual wear" , "Loose casual trousers" .)