dear i am explaining it according to my assessment....someone may better know
1.putting somebody to litmus test means giving him/her some options to chose one of them according to his/her will.the options are mostly difficult to decide. Actually litmus test in science tells us about the acid and basis(both being opposite in traits)
so putting someone to litmus test means giving him opposing and thoughtprovoking choices. litmus test judges the abilities of that person by seeing his/her choice. clear?
2.he is undergoing an operation means he is under the influence of an operation.well, the meaning can be molded according to the situation.
undergo means coming under the influence of something or opting for something. clear?
3.well i have no clear idea regarding "clean up your acts" i speculate its meaning would be to clean all the wrongdoings done by one or you can use "redeem" as alternative. i speculate
4.overhead conditions" can also be guessed by scrutinising the whole sentence. its meanings may be difficult or critical conditions in some specific matter.
5.mainstream political leader means important and famous political leader like "benazir is mainstream political leader in pakistan"
6.to put somebody at the backfoot means degrading somebody by neglecting his/her role in any matter. i suppose that as i am not sured
bhai jan aur kuch pochna ho tu zaroor batana.i am always there for the service of my paki brothers.
Student or Learner