The first bit of pretentiousness upsets him
I believe the answer is:
peter
'him' is the definition.
'the first bit of pretentiousness upsets' is the wordplay.
'the first' becomes 'rete' (I can't justify this - if you can you should give a lot more credence to this answer).
'bit of' indicates taking the first letters.
'upsets' says the letters should be written in reverse.
The initial letter of 'pretentiousness' is 'p'.
'rete'+'p'='retep'
'retep' in reverse letter order is 'PETER'.
Can you help me to learn more?
(Other definitions for peter that I've seen before include "Leading apostle" , "Fellow < safe" , "One of Jesus' disciples" , "-- Sellers, comic actor" , "and 4 across Doctor Who actor" .)