Picture in periodical that is outside (5)
I believe the answer is:
image
'picture' is the definition.
('image' can be a synonym of 'picture')
'periodical that is outside' is the wordplay.
'periodical' becomes 'mag' (abbreviation for magazine).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'outside' means one lot of letters goes inside another.
'mag' inserted within 'ie' is 'IMAGE'.
'in' is the link.
(Other definitions for image that I've seen before include "Optical counterpart in mirror" , "Representation of person or thing" , "Likeness, could be spitting" , "Simulacrum" , "Likeness; statue" .)