Start to fall in French bog (3)
I believe the answer is:
fen
'bog' is the definition.
(thesaurus)
'start to fall in french' is the wordplay.
'start to' suggests taking the first letters.
'in french' becomes 'en' ('en' can be French for 'in' e.g. en France).
The initial letter of 'fall' is 'f'.
'f'+'en'='FEN'
(Other definitions for fen that I've seen before include "Marshland" , "Low-lying marshy area in England" , "Slough" , "Low marshy area" , "Low marsh" .)