Get back after disposing of one treacherous daughter (5)
I believe the answer is:
regan
'treacherous daughter' is the definition.
(King Lear character)
'get back after disposing of one' is the wordplay.
'get back' becomes 'regain' (I've seen this before).
'after disposing of' indicates named letters should be taken away.
'one' becomes 'i' (Roman numeral).
'regain' with 'i' taken away is 'REGAN'.
(Other definitions for regan that I've seen before include "One of King Lear's daughters" , "daughter seeking power" , "Lear's daughter" , "better half of Cornwall" , "Goneril's sister (K. Lear)" .)