A base is in operation (5)
I believe the answer is:
afoot
'in operation' is the definition.
('afoot' can mean taking place or in process)
'a base' is the wordplay.
'base' becomes 'foot' (synonyms).
'a'+'foot'='AFOOT'
'is' is the link.
(Other definitions for afoot that I've seen before include "Currently in progress" , "In the wind" , "Going on" , "Brewing" , "Astir" .)