Record crowd causes congestion (3,3)
I believe the answer is:
log jam
'congestion' is the definition.
(log jam is a type of blockage)
'record crowd' is the wordplay.
'record' becomes 'log' (logging is a kind of recording).
'crowd' becomes 'jam' (jam is a kind of crowd).
'log'+'jam'='LOG-JAM'
'causes' is the link.
(Other definitions for log jam that I've seen before include "Impasse" , "what prevents progress" , "hindrance to progress" , "Major stoppage" , "A deadlock or work accumulation" .)