Charlie replaces Snoopy's lead in tears (6)
I believe the answer is:
crying
'in tears' is the definition.
(someone crying has burst into tears)
I can't explain the remainder of the clue.
(Other definitions for crying that I've seen before include "Lamentable" , "Sobbing" , "Tearful" , "Weeping" , "Lachrymose" .)