Had sunk, without exception, into the morass (6)
I believe the answer is:
fallen
'had sunk' is the definition.
I can't judge whether this defines the answer.
'without exception into the morass' is the wordplay.
'without exception' becomes 'all' (I've seen this before).
'into' indicates putting letters inside.
'the morass' becomes 'fen' (both can mean a boggy area).
'all' inserted inside 'fen' is 'FALLEN'.
Can you help me to learn more?
(Other definitions for fallen that I've seen before include "Immoral" , "Seduced" , "Pedal collapse resulting in flat feet" , "disgraced" , "dead" .)