Boxer appearing in tabloid periodically (3)
I believe the answer is:
ali
'boxer' is the definition.
(Muhammad Ali)
'tabloid periodically' is the wordplay.
'periodically' indicates alternate letters (letters must be selected 'every so often').
The alternate letters of 'tabloid' are 'ALI'.
'appearing in' is the link.
(Other definitions for ali that I've seen before include "Muhammad's son-in-law" , "Person" , "Son-in-law to Muhammad" , "A champ'" , "Old fighter" .)