Base member beginning to yatter in song of mourning (5)
I believe the answer is:
elegy
'song of mourning' is the definition.
(I've seen this in another clue)
'base member beginning to yatter' is the wordplay.
'base' becomes 'e' (Euler's number - base of natural logarithms).
'member' becomes 'leg' (member can mean a limb).
'beginning to' suggests taking the first letters.
The initial letter of 'yatter' is 'y'.
'e'+'leg'+'y'='ELEGY'
'in' acts as a link.
(Other definitions for elegy that I've seen before include "Plaintive poem" , "A sorrowful poem - Gray wrote a famous one" , "Song of lamentation" , "Poem of mourning" , "Mournful verse" .)