About to assess old plane (5)
I believe the answer is:
crate
'old plane' is the definition.
(informal term for an old vehicle)
'about to assess' is the wordplay.
'about' becomes 'c' (circa).
'to assess' becomes 'rate' (rating is a kind of assessing).
'c'+'rate'='CRATE'
(Other definitions for crate that I've seen before include "Box for shipping" , "Clapped-out vehicle" , "Wooden container" , "heap" , "Old soldiers might be found in this" .)