Little son carrying weight (10)
I believe the answer is:
smattering
'little' is the definition.
(I know that little can be written as smattering)
'son carrying weight' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'carrying weight' becomes 'mattering' (to carry weight is to matter or be important).
's'+'mattering'='SMATTERING'
(Other definitions for smattering that I've seen before include "little knowledge" , "Little bit" , "Taste" , "bit of learning" , "Tiny amount (of knowledge)" .)