VIPs with supporters in T-shirts? (3,5)
I believe the answer is:
top brass
'vips' is the definition.
(both can mean important or senior people)
'supporters in t-shirts?' is the wordplay.
'supporters' becomes 'bras' (a bra gives support to breasts).
'in' means one lot of letters goes inside another.
't-shirts?' becomes 'tops' (T-shirt is a type of top).
'bras' placed inside 'tops' is 'TOP BRASS'.
'with' is the link.
(Other definitions for top brass that I've seen before include "senior command" , "People in authority (informal)" , "High-ranking figures of authority" , "VIPs" , "high-ranking officials" .)