Fluid gender, we start to suspect, in marsh bird (5,4)

'marsh bird' is the definition.
'sedge wren' can be an answer for 'bird' (sedge wren is a kind of bird). I am unsure of the 'marsh' bit.

'fluid gender we start to suspect' is the wordplay.
'fluid' is an anagram indicator.
'start to' indicates taking the first letters.
The initial letter of 'suspect' is 's'.
'gender'+'we'+'s'='genderwes'
'genderwes' anagrammed gives 'SEDGE-WREN'.

'in' acts as a link.

Can you help me to learn more?