The old thus stopping before monstrous carbuncle (7)
I believe the answer is:
eyesore
'monstrous carbuncle' is the definition.
'eyesore' can be an answer for 'carbuncle' (I've seen this before). I am not certain of the 'monstrous' bit.
'the old thus stopping before' is the wordplay.
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'thus' becomes 'so' (synonyms).
'stopping' means one lot of letters goes inside another (inserted letters stop or plug a gap).
'before' becomes 'ere' (poetic term).
'ye'+'so'='yeso'
'yeso' going into 'ere' is 'EYESORE'.
Can you help me to learn more?
(Other definitions for eyesore that I've seen before include "Something exceedingly ugly" , "Something ugly to look at" , "bad sight" , "Blemish" , "Blot on the landscape" .)