Discolourations, we hear, on the map of Surrey (7)
I believe the answer is:
staines
'the map of surrey' is the definition.
I can't judge whether this definition defines the answer.
'discolourations we hear' is the wordplay.
I cannot really see how this works, but
'we hear' could be 'e' (I've seen this in other clues) and 'e' is found within the answer.
'discolourations' could be 'stains' ('stain' can be a synonym of 'discolouration') and 'stains' is found within the leftover letters.
No letters remain.
This explanation may well be incorrect...
'on' is the link.
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" .)