Sack with gold brick's first found in coat (6)
I believe the answer is:
bedaub
'coat' is the definition.
(thesaurus)
'sack with gold brick's first' is the wordplay.
'sack' becomes 'bed' ('in the sack' means 'in bed'**).
'with' is a charade indicator (letters next to each other).
'gold' becomes 'au' (Au is the chemical symbol for gold).
'first' says to take the initial letters.
The initial letter of 'bricks' is 'b'.
'bed'+'au'+'b'='BEDAUB'
'found in' acts as a link.
(Other definitions for bedaub that I've seen before include "Smear (with a sticky substance)" , "Crudely paint" , "Smear with something sticky" , "Splodge with paint" .)