Furtive behaviour of the last out? (7)
I believe the answer is:
stealth
'furtive behaviour' is the definition.
(I've seen this before)
'the last out?' is the wordplay.
'out?' indicates an anagram (out can mean wrong or inaccurate).
'the'+'last'='thelast'
'thelast' with letters rearranged gives 'STEALTH'.
'of' acts as a link.
(Other definitions for stealth that I've seen before include "Cats display this" , "Kind of indirect tax" , "Quiet or secret way of behaviour" , "Low-visibility military technology" , "Unseen machination" .)