Short cone and tall cone share an end (8)
I believe the answer is:
conspire
'end' is the definition.
Although both the answer and definition are verbs in their base form, I don't see how they can define each other.
'short cone and tall cone share' is the wordplay.
'short' means to remove the last letter.
'and' means one lot of letters go next to another.
'tall cone share' becomes 'spire' (I am not sure about this - if you are sure you should believe this answer much more).
'cone' with its last letter taken away is 'con'.
'con'+'spire'='CONSPIRE'
'an' acts as a link.
Can you help me to learn more?
(Other definitions for conspire that I've seen before include "'Plot, connive (8)'" , "Plot together - on prices (anag)" , "So prince is given to plan secretly with others" , "Secretly plan together" , "Collude" .)