Son leaving highlights in hair (7)
I believe the answer is:
tresses
'hair' is the definition.
(tresses are locks of hair)
'son leaving highlights' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'leaving' suggests deleting specific letters.
'highlights' becomes 'stresses' (I've seen this in another clue).
'stresses' with 's' removed is 'TRESSES'.
'in' acts as a link.
(Other definitions for tresses that I've seen before include "Long braids of hair" , "Long locks of hair" , "The hair of the head" .)