Lay out trendy waistcoat (6)
I believe the answer is:
invest
'lay out' is the definition.
(thesaurus)
'trendy waistcoat' is the wordplay.
'trendy' becomes 'in'.
'waistcoat' becomes 'vest' ('vest' can be a synonym of 'waistcoat').
'in'+'vest'='INVEST'
(Other definitions for invest that I've seen before include "Lay siege" , "Eg, buy stocks and shares" , "Put money in to make a profit" , "Contribute money in hope of a return" , "Put money into business for profit" .)