One may have a right to gain estate, first and last (7)

'one may' is the definition.
The definition suggests a singular noun which matches the answer.

'a right to gain estate first and last' is the wordplay.
'right' becomes 'lien' (legal right).
'to gain' says to put letters next to each other.
'first and last' suggests removing the centre (only the first and last letters are used).
'estate' with its middle removed is 'ee'.
'a'+'lien'+'ee'='ALIENEE'

'have' is the link.

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