Sad end of mammal in calamitous ice age (7)
I believe the answer is:
elegiac
'sad' is the definition.
(I've seen this before)
'end of mammal in calamitous ice age' is the wordplay.
'end of' indicates one should take the final letters.
'in' is an insertion indicator.
'calamitous' indicates an anagram.
The last letter of 'mammal' is 'l'.
'ice'+'age'='iceage'
'iceage' is an anagram of 'eegiac'.
'l' going into 'eegiac' is 'ELEGIAC'.
(Other definitions for elegiac that I've seen before include "Expressing poetic sorrow for something past" , "Mournful, lamenting" , "Mournful like a sorrowful poem" , "Plaintive" , "Mournful or sad as poem may be" .)