Speechless that trendy's admitted to flash (6)
I believe the answer is:
minute
'flash' is the definition.
(thesaurus)
'speechless that trendy's admitted' is the wordplay.
'speechless' becomes 'mute' (I've seen this before).
'that trendy' becomes 'in'.
'admitted' indicates putting letters inside.
'mute' enclosing 'in' is 'MINUTE'.
'to' is the link.
(Other definitions for minute that I've seen before include "Barely perceptible" , "Short period of time" , "Tiny like time unit (6)" , "Extremely small" , "A few ticks" .)