Fibrous plant is invading South American lake (5)
I believe the answer is:
sisal
'fibrous plant' is the definition.
'is invading south american lake' is the wordplay.
'invading' means one lot of letters goes inside another.
'south american' becomes 'sa'.
'lake' becomes 'l'.
'is' placed into 'sa' is 'sisa'.
'sisa'+'l'='SISAL'
(Other definitions for sisal that I've seen before include "Plant producing rope" , "Rope fibre from leaves of tropical plant" , "Plant fibre used for ropes" , "Natural fibre" , "Agave fibre found in bad sails" .)