Set of beliefs about marsh plant (5)
I believe the answer is:
creed
'set of beliefs' is the definition.
(I've seen this before)
'about marsh plant' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'marsh plant' becomes 'reed' (I've seen this before**).
'c'+'reed'='CREED'
(Other definitions for creed that I've seen before include "Set of beliefs or principles" , "Church doctrine" , "Statement of belief" , "Statement of beliefs" , "teleprinter" .)