Air-gun ad put out in newspaper (8)
I believe the answer is:
guardian
'newspaper' is the definition.
(UK newspaper)
'air-gun ad put out' is the wordplay.
'air gun' becomes 'airgun' (synonyms).
'put out' indicates anagramming the letters.
'airgun'+'ad'='airgunad'
'airgunad' is an anagram of 'GUARDIAN'.
'in' is the link.
(Other definitions for guardian that I've seen before include "Paper, protector" , "Protector or custodian" , "Protector or watchful angel" , "Champion - paper" , "Defender, protector" .)