Stains seen a great deal at the Boar's Head (5)

I believe the answer is:

blots

'stains' is the definition.
('blot' can be a synonym of 'stain')

'a great deal at the boar's head' is the wordplay.
'a great deal' becomes 'lots' (I've seen this in another clue).
'at' says to put letters next to each other (I've seen this in other clues).
'head' indicates taking the first letters.
The initial letter of 'boars' is 'b'.
'lots' after 'b' is 'BLOTS'.

'seen' acts as a link.

(Another definition for blots that I've seen is " Stains, perhaps on copybook".)

I've seen this clue in the Evening Standard.

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