Cautious about empty words in guarantee (8)
I believe the answer is:
warranty
'guarantee' is the definition.
(I know that guarantee can be written as warranty)
'cautious about empty words' is the wordplay.
'cautious' becomes 'wary' (similar in meaning).
'about' indicates putting letters inside.
'empty words' becomes 'rant' (a rant might be full of empty words).
'wary' placed around 'rant' is 'WARRANTY'.
'in' acts as a link.
(Other definitions for warranty that I've seen before include "cover provided" , "Certificate" , "assurance" , "Bond" , "Contract" .)