Heart-rending depression ultimately ate into us (7)
I believe the answer is:
piteous
'heart-rending' is the definition.
(I've seen this before)
'depression ultimately ate into us' is the wordplay.
'depression' becomes 'pit' (pit is a kind of depression).
'ultimately' suggests the final letters.
The last letters of 'ate into' are 'eo'.
'pit'+'eo'+'us'='PITEOUS'
(Other definitions for piteous that I've seen before include "Deserving compassion" , "Deserving sympathy" , "Woeful" , "Arousing sorrow and compassion" , "Heart-rending" .)