Activator, oddly inactive for so long (4)
I believe the answer is:
ciao
'so long' is the definition.
('goodbye' in Italian)
'activator oddly inactive' is the wordplay.
'oddly inactive' means one should take alternating letters (letters in odd-numbered positions are removed).
The alternate letters of 'activator' are 'CIAO'.
'for' is the link.
(Other definitions for ciao that I've seen before include "farewell" , "See you later in Rome?" , "Hello and goodbye to the Italians" , "Italian's so long" , "Informal greeting (Italian)" .)