That is hard, turning back only us, sick, in a very bad way (9)

I believe the answer is:

heinously

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'in a very bad way' is the definition.
I can't tell whether this definition defines the answer.

'that is hard turning back only us sick' is the wordplay.
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'turning back' is a reversal indicator.
'sick' indicates an anagram.
'ie'+'h'='ieh'
'ieh' written backwards gives 'hei'.
'only'+'us'='onlyus'
'onlyus' is an anagram of 'nously'.
'hei'+'nously'='HEINOUSLY'

Can you help me to learn more?

(Another definition for heinously that I've seen is " In a wicked and odious manner".)

I've seen this clue in The Independent.

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