Popular old routine? Not accurate (7)
I believe the answer is:
inexact
'not accurate' is the definition.
(I've seen this before)
'popular old routine?' is the wordplay.
'popular' becomes 'in'.
'old' becomes 'ex'.
'routine?' becomes 'act' (routine is a kind of act).
'in'+'ex'+'act'='INEXACT'
(Other definitions for inexact that I've seen before include "Not precise, like some sciences" , "Lax" , "Not entirely accurate" , "Not quite right" , "Not precise, ambiguous" .)