Highlight son's hair (6)
I believe the answer is:
stress
'highlight' is the definition.
(thesaurus)
'son's hair' is the wordplay.
'hair' becomes 'tress' (tress is a kind of hair).
's'+'tress'='STRESS'
(Other definitions for stress that I've seen before include "Strain or accentuate" , "Cause of crack" , "Emphasise the pressure" , "Emphasize" , "'Strain, tension (6)'" .)