100, old? Shut up! (5)
I believe the answer is:
caged
'shut up' is the definition.
(I've seen this in another clue)
'100 old?' is the wordplay.
'a00' becomes 'C' (Roman numeral).
'old?' becomes 'aged' ('aged' can be similar in meaning to 'old').
'c'+'aged'='CAGED'
(Other definitions for caged that I've seen before include "In a zoo?" , "Shut up" , "in prison" , "Confined behind bars" , "Cooped up" .)