Copy that is sent round small periodical (5)
I believe the answer is:
image
'copy' is the definition.
(an image might be a copy of something else)
'that is sent round small periodical' is the wordplay.
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'sent round' indicates putting letters inside.
'small periodical' becomes 'mag' (abbreviation for magazine; 'small' suggests a short form).
'ie' enclosing 'mag' is 'IMAGE'.
(Other definitions for image that I've seen before include "Likeness, could be spitting" , "Mental picture; metaphor" , "External impression" , "Copy" , "Representation, picture" .)