Unpaid workers sadly lost most of revenue (10)

'unpaid workers' is the definition.
('volunteer' can be a synonym of 'unpaid worker')

'sadly lost most of revenue' is the wordplay.
'sadly' indicates an anagram.
'most of' means to remove the last letter.
'revenue' with its last letter taken off is 'revenu'.
'lost'+'revenu'='lostrevenu'
'lostrevenu' with letters rearranged gives 'VOLUNTEERS'.