Pope modified line when among simple marine folk (9)

'marine folk' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.

'pope modified line when among simple' is the wordplay.
'modified' is an anagram indicator.
'line when' becomes 'l' (used when specifying particular lines from a poem. I am not sure about the 'when' bit.).
'among' is an insertion indicator (inserted letters are placed among others).
'simple' becomes 'mere' (synonyms).
'pope' with letters rearranged gives 'peop'.
'peop'+'l'='peopl'
'peopl' going into 'mere' is 'MERPEOPLE'.

Can you help me to learn more?