Guarantee being guarded taking on tirade (8)
I believe the answer is:
warranty
'guarantee' is the definition.
(synonyms)
'guarded taking on tirade' is the wordplay.
'guarded' becomes 'wary' (I've seen this before).
'taking on' means one lot of letters goes inside another.
'tirade' becomes 'rant' (I've seen this before).
'wary' placed around 'rant' is 'WARRANTY'.
'being' acts as a link.
(Other definitions for warranty that I've seen before include "Guarantee given to a purchaser" , "assurance" , "pledge" , "Quality guarantee" , "Bond" .)