Absent, a system (4)
I believe the answer is:
away
'absent' is the definition.
(similar in meaning)
'a system' is the wordplay.
'system' becomes 'way' (I can't explain this - if you can you should give a lot more credence to this answer).
'a'+'way'='AWAY'
Can you help me to learn more?
(Other definitions for away that I've seen before include "Having started" , "At a distance; match" , "See 1" , "Not at home, somewhere else" , "Not present, having left" .)