Thought processes lead to salt, perhaps, being replaced by bit of relish (9)
I believe the answer is:
reasoning
'thought processes' is the definition.
(someone's reasoning is their thought process)
I cannot understand how the remainder of the clue works.
(Other definitions for reasoning that I've seen before include "Logic" , "Train of thought" , "argument" , "Thinking that is coherent and logical" , "Debating" .)