Come first, keeping most of this inside (6)
I believe the answer is:
within
'inside' is the definition.
('within' can be a synonym of 'inside')
'come first keeping most of this' is the wordplay.
'come first' becomes 'win' (I've seen this before).
'keeping' indicates putting letters inside.
'most of' means to remove the last letter.
'this' with its last letter removed is 'thi'.
'win' going around 'thi' is 'WITHIN'.
(Other definitions for within that I've seen before include "Not going beyond" , "Don't let it go beyond this room!" , "Inside, enclosed by" , "Internally" , "Indoors" .)