Missing being idle when son’s away (7)
I believe the answer is:
lacking
'missing' is the definition.
(I know that missing can be written as lacking)
'being idle when son's away' is the wordplay.
I cannot really understand how this works, but
'being' could be 'king' (king is a kind of being) and 'king' is present in the answer.
'away' could be 'a' (abbreviation as used in sports fixtures) and 'a' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for lacking that I've seen before include "Missing" , "Deficient or inadequate" , "Short" , "Being without, wanting" , "'Deficient, wanting (7)'" .)