Deceitful behaviour in the last suspect (7)
I believe the answer is:
stealth
'deceitful behaviour' is the definition.
'the last suspect' is the wordplay.
'suspect' indicates anagramming the letters (a suspicious or unusual spelling).
'the'+'last'='thelast'
'thelast' with letters rearranged gives 'STEALTH'.
'in' is the link.
(Other definitions for stealth that I've seen before include "Cats display this" , "Skill in furtive activity" , "Unseen machination" , "Kind of indirect tax" , "Halt set in furtive manner" .)