That chap, beset by excess weight, taking up interest (6)
I believe the answer is:
behalf
'up interest' is the definition.
'behalf' can be an answer for 'interest' (I've seen this in another clue). I am unsure of the 'up' bit.
'that chap beset by excess weight' is the wordplay.
'that chap' becomes 'Hal'.
'beset by' is an insertion indicator ('beset' could historically mean 'cover').
'excess weight' becomes 'bef' (I can't justify this - if you can you should give a lot more credence to this answer).
'hal' inserted into 'bef' is 'BEHALF'.
'taking' is the link.
Can you help me to learn more?
(Other definitions for behalf that I've seen before include "Sake, interest, benefit" , "(A person's) interest" , "(Someone's) benefit or sake" , "On ...... of; as reps of" , "(On someone's) account" .)