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Third of relish in sarnies, awfully dry (8)

Ross

I believe the answer is:

rainless

I'm a little stuck... Click here to teach me more about this clue!

'dry' is the definition.
('rainless' can be similar in meaning to 'dry')

'third of relish in sarnies awfully' is the wordplay.
I cannot really see how this works, but
'in' is found in the answer.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?

(Another definition for rainless that I've seen is " when it's dry".)

I've seen this clue in The Telegraph.
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