Plant beginning thoroughly to overwhelm far side of swamp (9)
I believe the answer is:
speedwell
'plant' is the definition.
(speedwell is a kind of plant)
'beginning thoroughly to overwhelm far side of swamp' is the wordplay.
'beginning' becomes 'seed' (seed is a kind of beginning).
'thoroughly' becomes 'well' (I've seen this in another clue).
'to overwhelm' indicates putting letters inside.
'far side of' says to take the final letters.
The last letter of 'swamp' is 'p'.
'seed'+'well'='seedwell'
'seedwell' going around 'p' is 'SPEEDWELL'.
(Other definitions for speedwell that I've seen before include "Small plant, veronica" , "Small blue flower of the genus veronica" , "Flowering plant" , "bloomer?" , "Small creeping herbaceous plant with blue or pink flowers" .)