Home Office careless, missing first scam (4)
I believe the answer is:
hoax
'scam' is the definition.
(I've seen this before)
'home office careless missing first' is the wordplay.
'home office' becomes 'HO' (abbreviation).
'careless' becomes 'lax' (synonyms).
'missing first' means to remove the first letter.
'lax' with its initial letter taken away is 'ax'.
'ho'+'ax'='HOAX'
(Other definitions for hoax that I've seen before include "Fool" , "Kid" , "A plan to deceive" , "cod" , "Malicious act" .)