Smell gas, initially put down as marsh (3)
I believe the answer is:
bog
'marsh' is the definition.
(a bog is a marshy area)
'smell gas initially' is the wordplay.
'smell' becomes 'B.O.' (informal abbreviation for body odour).
'initially' indicates taking the first letters.
The initial letter of 'gas' is 'g'.
'bo'+'g'='BOG'
'put down as' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for bog that I've seen before include "Swamp" , "Wet spongy land" , "Soft wet land" , "Quagmire" , "Peaty wetland" .)