Signs of relief that may extend over a large area (7,5)
I believe the answer is:
contour lines
'signs' is the definition.
(contour line is a kind of sign)
'of relief that may extend over a large area' is the wordplay.
I cannot quite see how this works, but
'of' could be 'o' and 'o' is found in the answer.
'over' could be 'o' (cricket abbreviation) and 'o' is located in the answer.
'large' could be 'l' (eg in clothes sizes) and 'l' is present in the answer.
'area' could be 'turin' (Turin is an example) and 'turin' is present in the leftover letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for contour lines that I've seen is " Features of maps".)