Suit maker trimmed formal coat with gold (6)
I believe the answer is:
tailor
'suit maker' is the definition.
(I've seen this before)
'trimmed formal coat with gold' is the wordplay.
'trimmed formal coat' becomes 'tail' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'with' says to put letters next to each other.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'tail'+'or'='TAILOR'
Can you help me to learn more?
(Other definitions for tailor that I've seen before include "Suit maker" , "Maker of men's suits" , "follower of tinker" , "Maker of fitted clothes" , "Seamster" .)