Acclaim actor: judge with fulsomeness capital part involving many lines (7,8)
I believe the answer is:
'capital part involving many lines' is the definition.
(London junction passed through by many train lines)
'acclaim actor judge with fulsomeness' is the wordplay.
'acclaim' becomes 'clap' (synonyms).
'actor' becomes 'ham' (ham actor is a kind of actor).
'judge with' becomes 'j' (abbreviation for judge. I am not sure about the 'with' bit.).
'fulsomeness' becomes 'unction' (synonyms).
Can you help me to learn more?
(Other definitions for clapham junction that I've seen before include "Stop (for 6) [TRAINS UP]" , "Popular stop" .)