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