London giant’s journey featured in periodical (5)
I believe the answer is:
magog
'london giant's' is the definition.
(mythical figure said to guard London)
'journey featured in periodical' is the wordplay.
'journey' becomes 'go' (I've seen this in other clues).
'featured in' indicates putting letters inside.
'periodical' becomes 'mag' (abbreviation for magazine).
'go' placed within 'mag' is 'MAGOG'.
(Other definitions for magog that I've seen before include "Gog's companion" , "Gog and -, City of London guardian giants" , "Biblical king" , "Last of two old giants" , "old giant" .)