With regard to the two clues being debated:
5a Frustrate tot (6)
Chamber's definitions include:
* Scotch whisky, or a glass of it
* to frustrate, put an end to
6d Pretence of many a worker (4)
* a hypocritical, affected or perfunctory style of speech or thought
I accept that, in the second clue, the use of cant for pretence is not the first thing that comes to mind, but it is within the bounds of what you must expect in a cryptic crossword.
Caravaggio said that he had checked various dictionaries etc. - well it has been said before, Chambers is the preferred dictionary for all Telegraph crosswords, incuding the devilishly evil Enigmatic Variations. If you choose to do the Telegraph crosswords without access to a copy then you will always have problems like those above. It's a case of if you can't beat them then you have to join them.