Dirty spots in part of plane, craft beset by bacteria (11)

I believe the answer is:

fingermarks

I'm a little stuck... Click here to teach me more about this clue!

'dirty spots' is the definition.
'fingermarks' can be an answer for 'spots' (fingermark is a kind of spot). I am not sure about the 'dirty' bit.

'in part of plane craft beset by bacteria' is the wordplay.
'in part of plane' becomes 'fin' (I can't explain this - if you can you should believe this answer much more).
'craft' becomes 'ark' (ark is a kind of craft).
'beset by' is an insertion indicator ('beset' could historically mean 'cover').
'bacteria' becomes 'germs' (I've seen this in another clue).
'ark' put inside 'germs' is 'germarks'.
'fin'+'germarks'='FINGERMARKS'

Can you help me to learn more?

I've seen this clue in The Telegraph.

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