Writing-fluid half-hearted leftist has abandoned quietly (3)
I believe the answer is:
ink
'writing-fluid' is the definition.
(fluid for writing with)
'half-hearted leftist has abandoned quietly' is the wordplay.
'half-hearted leftist' becomes 'pink' (pink can mean somewhat leftwing).
'has abandoned' indicates named letters should be taken away.
'quietly' becomes 'p' (p for piano and quiet).
'pink' with 'p' taken out is 'INK'.
(Other definitions for ink that I've seen before include "Writing material" , "Cuttlefish defence" , "Liquid in pens" , "Printer's medium" , "Liquid used to write" .)