Think of one periodical in English (7)
I believe the answer is:
imagine
'think of' is the definition.
(thesaurus)
'one periodical in english' is the wordplay.
'one' becomes 'i' (Roman numeral).
'periodical' becomes 'mag' (abbreviation for magazine).
'english' becomes 'e' (abbreviation).
'i'+'mag'+'in'+'e'='IMAGINE'
(Other definitions for imagine that I've seen before include "Form a mental image in the mind" , "Conjure up in mind,fancy" , "1971 John Lennon song" , "Conjure up in mind, visualise" , "John Lennon classic (just think of it!)" .)