Smart little son sitting on lap (5)
I believe the answer is:
slick
'smart' is the definition.
(I've seen this before)
'little son sitting on lap' is the wordplay.
'little son' becomes 's' (abbreviation for son).
'sitting on' means one lot of letters go next to another.
'lap' becomes 'lick' ('lick' can be a synonym of 'lap').
's'+'lick'='SLICK'
(Other definitions for slick that I've seen before include "Glibly assured" , "it's very oily" , "Sleek - film of oil" , "Showing Polish" , "Unwanted floater" .)