Genius 243 (Guardian)

[Unicorn5]

Wordplay in each Across clue contians a superfluous letter.

I think I have finished this crossword, but cannot parse the following. Any help appreciated.

25A Nasty op trauma that's admitted to surgeon at times (9). I have AMPUTATOR which is an anagram of op and trauma, but with a spare T. The superfluous letter should be an A, so I'm confused.

29A Former pop singer snipping tail from irritated ruminant (7). I have ORBISON which contains the ruminant (bison), but how is the rest parsed. The superfluous letter should be an S.

26D American's over most of the rugby (4) THRU (def is the US for 'over', I think, but what about the parsing?)

27A Rounded shapes I toss overhead? (4) LOCI? (def is 'rounded shapes', I think, but again I can't understand the parsing.

Many thanks for any help.

[Neveracrossword]

29 Sore would be 'irriated', drop the E from the end - and the S is the superfluous letter.

[Neveracrossword]

26 Th = most of The +RU =  rugby union.

[Unicorn5]

Ah, yes! Thank you NAC.

[Neveracrossword]

27 Lobi, rounded shapes. Lob, toss, with I, in clue first, overhead.

[ProfessorMaisie]

For 25a the anag is 'OP TRAUMA' but with 'T[o]' inside, minus the extra 'o'.

[Unicorn5]

Thanks very much both. All finished now :)