Ruse devised by informer in theatre musical to begin with (9)
I believe the answer is:
stratagem
'ruse' is the definition.
(I know that stratagem is a type of fraud)
'devised by informer in theatre musical to begin with' is the wordplay.
I cannot quite see how this works, but
'by' could be 'at' and 'at' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for stratagem that I've seen before include "Tag master with cunning plan" , "A cunning plan or piece of trickery" , "Devious ploy" , "Cunning plot" , "Ruse" .)