Unfair offer securing most of a good thing (7)
I believe the answer is:
biassed
'unfair' is the definition.
(something with a bias is unfair or unequal)
'offer securing most of a good thing' is the wordplay.
'offer' becomes 'bid' (synonyms).
'securing' means one lot of letters goes inside another.
'most of' means to remove the last letter.
'a good thing' becomes 'asset' (an advantageous or desirable thing).
'asset' with its last letter taken off is 'asse'.
'bid' placed around 'asse' is 'BIASSED'.
(Another definition for biassed that I've seen is " Prejudiced".)