Daughter late fixing mouth (5)
I believe the answer is:
delta
'mouth' is the definition.
(I know that delta is a type of mouth)
'daughter late fixing' is the wordplay.
'daughter' becomes 'd' (genealogical abbreviation).
'fixing' indicates anagramming the letters.
'late' is an anagram of 'elta'.
'd'+'elta'='DELTA'
(Other definitions for delta that I've seen before include "sort of wing" , "Fragmented estuary" , "Area of distributaries in a river basin" , "See Greek" , "River mouth in the Greek alphabet" .)