Socket in bum provides explanation for absence (4,4)
I believe the answer is:
sick note
'explanation for absence' is the definition.
I know nothing about this answer so I can't tell whether this works.
'socket in bum' is the wordplay.
'bum' is an anagram indicator (bum can mean faulty or wrong).
'socket'+'in'='socketin'
'socketin' is an anagram of 'SICK NOTE'.
'provides' acts as a link.
Can you help me to learn more?
(Other definitions for sick note that I've seen before include "Illness certificate" , "Doctor might supply this" .)