Old pens in one pocket (9)
I believe the answer is:
miniature
'pocket' is the definition.
(thesaurus)
'old pens in one' is the wordplay.
'old' becomes 'mature' (mature can mean old or grown-up).
'pens' means one lot of letters goes inside another (inserted letters are held or penned in).
'one' becomes 'i' (Roman numeral).
'in'+'i'='ini'
'mature' enclosing 'ini' is 'MINIATURE'.
(Other definitions for miniature that I've seen before include "Of a much smaller size than usual" , "Little" , "Diminutive" , "Smaller than normal" , "Putting game" .)