Transgression, dark’s ending in fall (4)
I believe the answer is:
sink
'fall' is the definition.
(sinking is a kind of falling)
'transgression dark's ending' is the wordplay.
'transgression' becomes 'sin' (sin is a kind of transgression).
'ending' indicates one should take the final letters.
The final letter of 'dark' is 'k'.
'sin'+'k'='SINK'
'in' is the link.
(Other definitions for sink that I've seen before include "Plumbing fixture" , "Plummet" , "Scene of iniquity or washing up" , "Basin - scupper" , "(Cause to) become submerged" .)