Keep out of sight, certain to be reactionary (9)
I believe the answer is:
hidebound
'be reactionary' is the definition.
(thesaurus)
'keep out of sight certain' is the wordplay.
'keep out of sight' becomes 'hide' (I've seen this before).
'certain' becomes 'bound' (as in the phrase 'it's bound to be').
'hide'+'bound'='HIDEBOUND'
'to' is the link.
(Other definitions for hidebound that I've seen before include "Narrow" , "Rigid" , "very conservative" , "Restricted by petty rules" , "Restricted by conventional ideas" .)