Porter carrying paintings set off (5,3)
I believe the answer is:
start out
'set off' is the definition.
('start out' can be a synonym of 'set off')
'porter carrying paintings' is the wordplay.
'porter' becomes 'stout' ('stout' can be a synonym of 'porter').
'carrying' means one lot of letters goes inside another.
'paintings' becomes 'art' (I've seen this before).
'stout' enclosing 'art' is 'START OUT'.
(Other definitions for start out that I've seen before include "launch" , "Set off" , "Take off" , "Go off" , "Begin a journey" .)