Popular 'Old Bill' type heading off... (7)
I believe the answer is:
inexact
'off' is the definition.
(both can mean imprecise)
'popular old bill type heading' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'old' becomes 'ex' (prefix meaning former or onetime).
'bill' becomes 'ac' (short for account).
'heading' says to take the initial letters (head could mean the top letter).
The initial letter of 'type' is 't'.
'in'+'ex'+'ac'+'t'='INEXACT'
(Other definitions for inexact that I've seen before include "Lax" , "not wholly Right" , "Woolly" , "Fuzzy, muddled" , "not precisely correct" .)