Architect catching large fish (8)
I believe the answer is:
flounder
'fish' is the definition.
(I know that flounder is a type of fish)
'architect catching large' is the wordplay.
'architect' becomes 'founder' (both can mean a pioneer or initiator).
'catching' indicates putting letters inside.
'large' becomes 'L' (eg in clothes sizes).
'founder' enclosing 'l' is 'FLOUNDER'.
(Other definitions for flounder that I've seen before include "Fish -- stumble helplessly" , "Grope around" , "Have difficulties with the flatfish" , "fumble?" , "Small flatfish" .)