Odd enough to start one ruminating (3)
I believe the answer is:
rum
'odd' is the definition.
(rum can mean strange or dubious)
'enough to start one ruminating' is the wordplay.
'enough' becomes 'um' (I can't justify this - if you can you should give a lot more credence to this answer).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'start' says to take the initial letters.
The first letter of 'ruminating' is 'r'.
'um' put after 'r' is 'RUM'.
Can you help me to learn more?
(Other definitions for rum that I've seen before include "Sugar-cane product" , "An alcoholic spirit - sounds odd" , "Strange; spirit" , "Odd sort of drink" , "Pirate's grog" .)