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 am not sure about this - if you are sure you should give a lot more credence to this answer).
'would' becomes 'd' (as in 'we'd' - 'we would').
'be sent round' is an insertion indicator.
'en'+'d'='end'
'sure' going into '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" .)