Son having a long way to go looks happy (6)
I believe the answer is:
smiles
'looks happy' is the definition.
(someone smiles when they are happy)
'son having a long way to go' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'having' means one lot of letters go next to another.
'a long way to go' becomes 'miles' (a long distance).
's'+'miles'='SMILES'
(Other definitions for smiles that I've seen before include "Looks happy" , "Facial expressions showing pleasure" , "expresses pleasure" , "Shows signs of pleasure or amusement" , "Beams, grins" .)