Reason second up-and-coming musical is cut short (6)
I believe the answer is:
motive
'reason' is the definition.
(I've seen this before)
'second up-and-coming musical is cut short' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'up-and-coming' is a reversal indicator.
'musical' becomes 'evita' (I've seen this before).
'is cut short' means to remove the last letter.
'evita' with its final letter taken away is 'evit'.
'evit' reversed gives 'tive'.
'mo'+'tive'='MOTIVE'
(Other definitions for motive that I've seen before include "Reason for doing something, say murder" , "Inspiration" , "Incitement to do something" , "Underlying purpose" , "Ground" .)