Board quickly reduced by US agency (6)
I believe the answer is:
fascia
'board' is the definition.
(I know that fascia is a type of board)
'quickly reduced by us agency' is the wordplay.
'quickly' becomes 'fast' (synonyms).
'reduced' means to remove the last letter.
'by' is a charade indicator (letters next to each other).
'us agency' becomes 'cia' (I've seen this before).
'fast' with its last letter removed is 'fas'.
'fas'+'cia'='FASCIA'
(Other definitions for fascia that I've seen before include "Flat surface (archit.)" , "board above shop window" , "panel" , "Detachable (mobile phone) front cover" , "Car dashboard" .)