Woman beginning to mourn first murder victim (5)
I believe the answer is:
mabel
'woman' is the definition.
(I've seen this before)
'beginning to mourn first murder victim' is the wordplay.
'beginning to' says to take the initial letters.
'first murder victim' becomes 'Abel' (Abel was murdered by his brother Cain in the Bible).
The initial letter of 'mourn' is 'm'.
'm'+'abel'='MABEL'
(Other definitions for mabel that I've seen before include "Lady" , "Blame (anag.), lady's name" , "Girl's name" , "Woman" , "Mack's mate" .)