The son went off and lumbered one with (7)
I believe the answer is:
saddled
'the son went' is the definition.
I can't tell whether this definition defines the answer.
'off and lumbered one with' is the wordplay.
'off' becomes 'addled' (I've seen this before).
'and' means one lot of letters go next to another.
'lumbered one with' becomes 's' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'addled' put after 's' is 'SADDLED'.
Can you help me to learn more?
(Other definitions for saddled that I've seen before include "stuck" , "Horse might be" , "Encumbered" , "Burdened with" , "Having to put up (with) - ready to be ridden" .)