Mum, getting terribly hot, ran in race (8)
I believe the answer is:
marathon
'race' is the definition.
(thesaurus)
'mum getting terribly hot ran' is the wordplay.
'mum' becomes 'ma' ('ma' can be a synonym of 'mum').
'getting' says to put letters next to each other.
'terribly' is an anagram indicator.
'hot'+'ran'='hotran'
'hotran' with letters rearranged gives 'rathon'.
'ma'+'rathon'='MARATHON'
'in' acts as a link.
(Other definitions for marathon that I've seen before include "Race; battle" , "Long-distance running event" , "Pheidippides' starting point" , "26-mile foot race" , "Race of about 26 miles" .)