This writer's breaking pound to get quarter (4)
I believe the answer is:
limb
'quarter' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'this writer's breaking pound' is the wordplay.
'this writer's' becomes 'I'm' ('I am' referring to the writer of the clue).
'breaking' indicates putting letters inside.
'pound' becomes 'lb' (abbreviation for the unit of weight).
'im' placed within 'lb' is 'LIMB'.
'to get' is the link.
Can you help me to learn more?
(Other definitions for limb that I've seen before include "arm, perhaps" , "Branch - member" , "as part of plane, say" , "Bough" , "Substantial branch from a tree trunk" .)