Lengthen lease, in writing (3,4)
I believe the answer is:
let down
'lengthen' is the definition.
(I've seen this before)
'lease in writing' is the wordplay.
'lease' becomes 'let' ('let' can be a synonym of 'lease').
'in writing' becomes 'down' (as in written down).
'let'+'down'='LET-DOWN'
(Other definitions for let down that I've seen before include "Leave in the lurch" , "Disappoint, betray" , "blow" , "Disappoint, fail to keep promise" , "deflating" .)