Leave unfinished stout by type of arch (6)
I believe the answer is:
gothic
'type of arch' is the definition.
'leave unfinished stout' is the wordplay.
I cannot really see how this works, but
'leave' could be 'go' and 'go' is present in the answer.
This may be the basis of clue (or it may be nonsense).
'by' acts as a link.
Can you help me to learn more?
(Other definitions for gothic that I've seen before include "Architectural or literary style" , "Building style of medieval cathedrals" , "Of architecture with flying buttresses, for example" , "Style of architecture popular in Europe in the 12th to 16th centuries" , "Describing some architecture" .)