Dad's not heavy on him! (6)
I believe the answer is:
father
'dad's' is the definition.
(I've seen this before)
'not heavy on him' is the wordplay.
'not heavy' becomes 'the' (I can't explain this - if you can you should believe this answer much more).
'on' is an insertion indicator.
'him' becomes 'far'.
'the' inserted within 'far' is 'FATHER'.
Can you help me to learn more?
(Other definitions for father that I've seen before include "Sire" , "pops" , "relation" , "and 13: Jolly gift-deliverer" , "Relative and priest" .)