Guaranteed a certain tip would be sent round (7)
I believe the answer is:
ensured
'guaranteed' is the definition.
('ensure' can be a synonym of 'guarantee')
'a certain tip would be sent round' is the wordplay.
'a certain' becomes 'sure' (I've seen this before).
'tip' becomes 'en' (I can't explain this - if you can you should believe this answer much more).
'would' becomes 'd' (I've seen this in other clues).
'be sent round' is an insertion indicator.
'en'+'d'='end'
'sure' going within 'end' is 'ENSURED'.
Can you help me to learn more?
(Other definitions for ensured that I've seen before include "Determined" , "Saw to it" , "Make certain of" , "Made certain for end user" , "guaranteed" .)