Nowadays, a new number to perform: 'Teeny-Weeny' (4,6)
I believe the answer is:
anno domini
'nowadays' is the definition.
'a new number to perform teeny-weeny' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'number' becomes 'no.' (abbreviation).
'to perform' becomes 'do' (both can mean complete an action).
'teeny-weeny' becomes 'mini'.
'a'+'n'+'no'+'do'+'mini'='ANNO DOMINI'
(Other definitions for anno domini that I've seen before include "Advancing old age (informal)" , "AD in full" , "later days" , "having a date, she may put it on" , "In the year of our Lord" .)