Reasonable cause to find place of entertainment (10)
I believe the answer is:
fairground
'place of entertainment' is the definition.
'reasonable cause' is the wordplay.
'reasonable' becomes 'fair' (associated in meaning).
'cause' becomes 'ground' ().
'fair'+'ground'='FAIRGROUND'
'to find' acts as a link.
(Other definitions for fairground that I've seen before include "Entertainment centre" , "Scary ride here perhaps" , "Entertainment venue" , "that holds attractions" , "good enough for entertainment" .)