Occasionally they upset build up (4)
I believe the answer is:
hype
'build up' is the definition.
(I've seen this before)
'occasionally they upset' is the wordplay.
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
'they'+'upset'='theyupset'
The alternate letters of 'theyupset' are 'HYPE'.
(Other definitions for hype that I've seen before include "Doer of dirty work" , "Over the top publicity" , "Extravagant promotion of a person or product" , "Slang for extravagant promotion of a product" , "Puffery" .)