Add 'fashionable' to 'clued-up (7)
I believe the answer is:
include
'add' is the definition.
(I know that include is a more specific form of the action add)
'fashionable to clued-up' is the wordplay.
'fashionable' becomes 'in' (in can mean fashionable or trendy).
'up' indicates anagramming the letters (letters get chucked up).
'clued' is an anagram of 'clude'.
'in'+'clude'='INCLUDE'
(Other definitions for include that I've seen before include "Admit" , "Allow participation in" , "Contain - embrace" , "Count in along with others" , "Contain as part of group" .)