It's bound to be a burden (4)
I believe the answer is:
load
'burden' is the definition.
(I know that burden can be written as load)
'it's bound to be a' is the wordplay.
'it's' becomes 'lod' (I can't justify this - if you can you should give a lot more credence to this answer).
'bound' is an insertion indicator (some letters are bound in others).
'to be a' becomes 'a'.
'lod' going around 'a' is 'LOAD'.
Can you help me to learn more?
(Other definitions for load that I've seen before include "Great quantity" , "Burden, onus" , "pack" , "Weight to be transported" , "that's put on commercial vehicle" .)