Suspiciously letting one off ain’t him (6)
I believe the answer is:
hitman
'suspiciously' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.
'letting one off ain't him' is the wordplay.
'letting' becomes 'h' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'one' becomes 'an' (an apple is one apple).
'off' is a deletion indicator (some letters taken off).
'him' becomes 'Man'.
'aint' with 'an' removed is 'it'.
'h'+'it'+'man'='HITMAN'
Can you help me to learn more?
(Other definitions for hitman that I've seen before include "Hired gun" , "offer?" , "killer" , "Assassin" .)