Available, decentralised, operating for a long time (4)
I believe the answer is:
aeon
'long time' is the definition.
(aeon is a kind of long time)
'available decentralised operating' is the wordplay.
'decentralised' means to remove the middle letters (the centre of the word is removed).
'operating' becomes 'on' (I've seen this in other clues).
'available' with its middle removed is 'ae'.
'ae'+'on'='AEON'
'for a' acts as a link.
(Other definitions for aeon that I've seen before include "A very long time, an age" , "Indefinitely long period of time" , "Eternity" , "With lots of time" , "Epoch" .)