Setter rolling in money mostly keeps working — not being serious! (6)
I believe the answer is:
ironic
'not being serious' is the definition.
'setter rolling in money mostly keeps working' is the wordplay.
'setter' becomes 'i' (the person that set the crossword).
'rolling in money' becomes 'rich'.
'mostly' means to remove the last letter.
'keeps' indicates putting letters inside.
'working' becomes 'on'.
'rich' with its final letter removed is 'ric'.
'ric' going around 'on' is 'ronic'.
'i'+'ronic'='IRONIC'
(Other definitions for ironic that I've seen before include "Being sarcastic or deliberately oblique" , "Strangely apt or inapt" , "'Wry, incongruous (6)'" , "Sarcastic or deliberately perverse" , "Meaning the contrary of what is expressed" .)