Guarantee that’s equally reliable (6)
I believe the answer is:
assure
'guarantee' is the definition.
('assure' can be a synonym of 'guarantee')
'equally reliable' is the wordplay.
'equally' becomes 'as' ('as' can be a synonym of 'equally').
'reliable' becomes 'sure' ('sure' can be similar in meaning to 'reliable').
'as'+'sure'='ASSURE'
'that's' acts as a link.
(Other definitions for assure that I've seen before include "Cover with a policy" , "Convince, guarantee" , "Guarantee, promise" , "Give confidence to" , "Make (something) certain to happen" .)