Highlight head of silver hair (6)
I believe the answer is:
stress
'highlight' is the definition.
(thesaurus)
'head of silver hair' is the wordplay.
'head of' indicates taking the first letters.
'hair' becomes 'tress' (tress is a kind of hair).
The first letter of 'silver' is 's'.
's'+'tress'='STRESS'
(Other definitions for stress that I've seen before include "result of being overworked" , "possible excuse for sickie?" , "Dwell on" , "Strain or accentuate" , "Tension; emphasis" .)