Rather terrible finish — it is distressing (12)
I believe the answer is:
heartrending
'distressing' is the definition.
(I've seen this before)
'rather terrible finish' is the wordplay.
'terrible' indicates an anagram.
'finish' becomes 'ending' (I've seen this before).
'rather' anagrammed gives 'heartr'.
'heartr'+'ending'='HEARTRENDING'
'it is' acts as a link.
(Other definitions for heartrending that I've seen before include "Agonising" , "causing great sadness" , "Distressing" , "Tragic" .)