Criticise the substance, being unhappy (10)
I believe the answer is:
discontent
'unhappy' is the definition.
('discontent' is associated in meaning with 'unhappy')
'criticise the substance' is the wordplay.
'criticise' becomes 'dis' (speak disrespectfully).
'the substance' becomes 'content' ('content' can be a synonym of 'substance'**).
'dis'+'content'='DISCONTENT'
'being' acts as a link.
(Other definitions for discontent that I've seen before include "'Now is the 16 of our _____ made 2 15 by this sun of 7'" , "Now is the winter of our - (Richard III)" , "Unhappiness" , "Now is the winter of our - (Rich.III)" , "Bad feelings" .)