Life story captures heart of miserable writer (4)
I believe the answer is:
biro
'writer' is the definition.
(type of pen, something which writes)
'life story captures heart of miserable' is the wordplay.
'life story' becomes 'bio' (short for biography).
'captures' means one lot of letters goes inside another.
'heart of' indicates the central letters.
The central letter of 'miserable' is 'r'.
'bio' going around 'r' is 'BIRO'.
(Other definitions for biro that I've seen before include "Writing implement" , "Lazlo -, ballpoint pen inventor" , "writer" , "Pocket item" , "Laszlo Jozsef -, ballpoint pen inventor" .)