Mark's heard somewhere in Surrey (7)
I believe the answer is:
staines
'mark's heard somewhere in surrey' is the definition.
I can't tell whether this definition defines the answer.
'mark's heard somewhere in' is the wordplay.
'mark's' becomes 'stain's' ('stain' can be a synonym of 'mark').
'heard somewhere' becomes 'e' (I can't justify this - if you can you should believe this answer much more).
'in' is an insertion indicator.
'stains' enclosing 'e' is 'STAINES'.
Can you help me to learn more?
(Other definitions for staines that I've seen before include "Surrey town on Thames" , "Town SW of Heathrow" .)