Cover up includes ultimate appeal to perform several jobs (9)
I believe the answer is:
multitask
'to perform several jobs' is the definition.
(I've seen this before)
'cover up includes ultimate appeal' is the wordplay.
'cover up' becomes 'mask'.
'includes' means one lot of letters goes inside another.
'ultimate' becomes 'ult'.
'appeal' becomes 'it'.
'ult'+'it'='ultit'
'mask' enclosing 'ultit' is 'MULTITASK'.
(Other definitions for multitask that I've seen before include "Do things simultaneously" , "execute jobs simultaneously" , "To perform several jobs" .)