Nick right to be upset with old addict (7)
I believe the answer is:
trouser
'nick' is the definition.
(both can mean to steal)
'right to be upset with old addict' is the wordplay.
'right' becomes 'rt' (abbreviation as in Rt Hon- the Right Honourable).
'to be upset' is a reversal indicator (I've seen 'being upset' mean this).
'with' says to put letters next to each other.
'old' becomes 'o' (abbreviation).
'addict' becomes 'user' (drug user).
'rt' written backwards gives 'tr'.
'tr'+'o'+'user'='TROUSER'
(Other definitions for trouser that I've seen before include "For oneself" , "Pocket (slang)" , "Pocket (something)" , "Our rest (anag)" , "Appropriate" .)