Double it up with an Italian master (6)
I believe the answer is:
titian
'italian master' is the definition.
'titian' can be an answer for 'master' (I have seen 'Old Master ' mean 'titian' so perhaps 'master' could also mean 'titian'). I'm not sure about the 'italian' bit.
'double it up with an' is the wordplay.
'double' indicates the doubling of some letters.
'up' is a reversal indicator (in down clue: letters go upwards).
'with' says to put letters next to each other.
'it' back-to-front is 'ti'.
'ti' duplicated is 'titi'.
'titi'+'an'='TITIAN'
Can you help me to learn more?
(Other definitions for titian that I've seen before include "Italian artist" , "The painter Tiziano Vecellio" , "Venetian artist" , "Italian painter; hair colour" , "Italian Old Master and hair-colour" .)