Affection for a swamp (4,4)
I believe the answer is:
soft spot
'affection' is the definition.
(I know that soft spot is a type of affection)
I don't understand how the rest of the clue works.
(Other definitions for soft spot that I've seen before include "showing fondness" , "Feeling of affection towards person - not hard" , "Emotional susceptibility" , "Sentimental fondness" , "Sentimental affection" .)