A tin's possible discolouration (5)

I believe the answer is:

stain

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

'discolouration' is the definition.
(synonyms)

'a tin's possible' is the wordplay.
'a tin's' anagrams to 'STAIN'.
However, I am not sure how the anagram is indicated.

Can you help me to learn more?

(Other definitions for stain that I've seen before include "Blotch on satin" , "Mark with colour, dirt" , "Tarnish" , "Discolour - blemish" , "Discoloured patch" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app