Minimum as written into lease (5)
I believe the answer is:
least
'minimum' is the definition.
(similar in meaning)
'as written into lease' is the wordplay.
'written into' is an insertion indicator.
'lease' becomes 'let' (synonyms).
'as' put inside 'let' is 'LEAST'.
(Other definitions for least that I've seen before include "Superlatively little" , "Most slight" , "superlatively small" , "Fewest" , "Opposite of most" .)