The source of the outfit I put on outside (6)
I believe the answer is:
origin
'the source' is the definition.
('origin' can be a synonym of 'source')
'outfit i put on outside' is the wordplay.
'outfit' becomes 'rig' (synonyms).
'put' means one lot of letters go next to another.
'outside' means one lot of letters goes inside another.
'rig'+'i'='rigi'
'rigi' going into 'on' is 'ORIGIN'.
'of the' acts as a link.
(Other definitions for origin that I've seen before include "Where X meets Y" , "Beginning (of species?)" , "extraction" , "spring" , "Genesis" .)