Lock (5)
'lock' is the definition.
(both can mean a piece of hair)
This is all the clue.
(Other definitions for tress that I've seen before include "Braid of hair" , "Long lock or plait of hair" , "Rests (anag.)" , "is a bit of a shock" , "Switch" .)
'lock' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for seal that I've seen before include "Make tight" , "A mammal; a fastening" , "Fasten securely; aquatic mammal" , "Secure; aquatic animal" , "Aquatic mammal - close securely" .)