Contour shows mother is out of shape (8)
I believe the answer is:
isotherm
'contour' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'mother is out of shape' is the wordplay.
'out of shape' indicates an anagram.
'mother'+'is'='motheris'
'motheris' is an anagram of 'ISOTHERM'.
'shows' is the link.
Can you help me to learn more?
(Other definitions for isotherm that I've seen before include "Line of equal heat" , "Line joining places of equal temperature" , "Curve of equal temperatures" , "Map line connecting places with same temperature" , "Weather map may show that" .)