Cases of trendy positions (9)
I believe the answer is:
instances
'cases' is the definition.
('instance' can be a synonym of 'case')
'trendy positions' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'positions' becomes 'stances' (stance is a kind of position).
'in'+'stances'='INSTANCES'
'of' acts as a link.
(Other definitions for instances that I've seen before include "Incessant (anag)" , "Examples, illustrations" .)