Addict: one put on kilo in a month (6)
I believe the answer is:
junkie
'addict' is the definition.
(junkie is a kind of addict)
'one put on kilo in a month' is the wordplay.
'one' becomes 'i' (Roman numeral).
'put on' means one lot of letters go next to another.
'kilo' becomes 'k' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' means one lot of letters goes inside another.
'a month' becomes 'june' (June is an example**).
'i' after 'k' is 'ki'.
'ki' put inside 'june' is 'JUNKIE'.
(Other definitions for junkie that I've seen before include "one shooting up" , "User" , "In slang, a drug addict" , "Jargon for a drug addict" .)