Graduate bumps into second person in marshy section of lake (5)
I believe the answer is:
bayou
'lake' is the definition.
(bayou is a kind of lake)
'graduate bumps into second person in marshy section' is the wordplay.
I cannot really understand how this works, but
'graduate' could be 'ba' (someone with a Bachelor of Arts degree) and 'ba' is present in the answer.
'second person' could be 'you' (second person pronoun) and 'you' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'of' is the link.
Can you help me to learn more?
(Other definitions for bayou that I've seen before include "Marshy area in southern US" , "Marshy offshoot of a river" , "AB, you could be in a marshy Louisiana inlet" , "US outlet" , "US marshland" .)