International publication for teachers holds 1000 articles (5)
I believe the answer is:
items
'articles' is the definition.
(I've seen this before)
'international publication for teachers holds 1000' is the wordplay.
'international' becomes 'i' (abbreviation eg in IMF).
'publication for teachers' becomes 'TES' (Times Educational Supplement).
'holds' is an insertion indicator.
'1000' becomes 'M' (roman numeral).
'tes' enclosing 'm' is 'tems'.
'i'+'tems'='ITEMS'
(Other definitions for items that I've seen before include "Individual articles" , "Individuals units" , "separate parts of news bulletin" , "Separate articles, as on list" , "Elements" .)