This pattern was formerly sufficient (7)
I believe the answer is:
example
'this pattern' is the definition.
(thesaurus)
'formerly sufficient' is the wordplay.
'formerly' becomes 'ex' (prefix meaning former or onetime).
'sufficient' becomes 'ample' (both can mean enough).
'ex'+'ample'='EXAMPLE'
'was' acts as a link.
(Other definitions for example that I've seen before include "Fact that illustrates a general rule" , "Illustration or model" , "Instance - pattern" , "Instance, specimen" , "Characteristic item" .)