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Seeing it sickens, I politely decline rather large number (3,1,6,5)

Ross

I believe the answer is:

not a pretty sight

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'seeing it sickens i politely decline rather large number' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'seeing it sickens i politely decline rather large number' is the wordplay.
I cannot quite see how this works, but
'seeing' could be 'sight' (I've seen this before) and 'sight' is found in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
'i' could be 'a' and 'a' is found within the answer.
'rather' could be 'pretty' (synonyms) and 'pretty' is found in the answer.
'number' could be 'no' (abbreviation for 'number') and 'no' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Another definition for not a pretty sight that I've seen is " badly knocked about".)

I've seen this clue in The Telegraph.
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