Since rein turned out to be hollow (9)
I believe the answer is:
insincere
'be hollow' is the definition.
(similar in meaning)
'since rein turned out' is the wordplay.
'turned out' indicates an anagram (turn out can mean to organise).
'since'+'rein'='sincerein'
'sincerein' with letters rearranged gives 'INSINCERE'.
'to' is the link.
(Other definitions for insincere that I've seen before include "is not ringing true" , "Dissembling" , "displaying hypocrisy" , "Feigned, fawning" , "may be laying it on" .)