Mug in musical kept up being unsuccessful (8)
I believe the answer is:
abortive
'being unsuccessful' is the definition.
('abortive' can be a synonym of 'unsuccessful')
'mug in musical kept up' is the wordplay.
'mug' becomes 'rob' (I've seen this in another clue).
'in musical' becomes 'evita' (Andrew Lloyd Webber musical. I am not sure about the 'in' bit.).
'kept' is an insertion indicator.
'up' is a reversal indicator (in down clue: letters go upwards).
'rob' going within 'evita' is 'evitroba'.
'evitroba' reversed gives 'ABORTIVE'.
Can you help me to learn more?
(Other definitions for abortive that I've seen before include "Failing to accomplish an intended result" , "O brave it and make it unsuccessful" , "Failing because done too soon" , "Useless" , "Coming to no result" .)