Sanction trendy underwear (6)
I believe the answer is:
invest
'sanction' is the definition.
The answer and definition can be both to do with social activities as well as being verbs in their base form.
Maybe you can see a link between them that I don't see?
'trendy underwear' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'underwear' becomes 'vest'.
'in'+'vest'='INVEST'
Can you help me to learn more?
(Other definitions for invest that I've seen before include "Put money into business for profit" , "Put money into a company" , "Put money into shares, property, etc" , "Contribute money in hopes of financial return" , "as Greeks did to Troy?" .)