Time invested in alternative campaign (9)
I believe the answer is:
operation
'campaign' is the definition.
(thesaurus)
'time invested in alternative' is the wordplay.
'time' becomes 'era' (an era is a period of time).
'invested in' is an insertion indicator.
'alternative' becomes 'option' (synonyms).
'era' going within 'option' is 'OPERATION'.
(Other definitions for operation that I've seen before include "working" , "Surgical or military piece of work" , "Manoeuvre - military action" , "battle" , "medical treatment" .)