Aim to plug old piece of furniture that can be lengthened (10)
I believe the answer is:
extendable
'that can be lengthened' is the definition.
'aim to plug old piece of furniture' is the wordplay.
'aim' becomes 'end' (I've seen this before).
'to plug' is an insertion indicator (to plug can mean to block a gap within something).
'old' becomes 'ex'.
'piece of furniture' becomes 'table' (table is a kind of piece of furniture).
'ex'+'table'='extable'
'end' put inside 'extable' is 'EXTENDABLE'.
(Another definition for extendable that I've seen is " designed to be pulled out".)