Obstinate Liberal, one into very soft stuff (6)
I believe the answer is:
mulish
'obstinate' is the definition.
(similar in meaning)
'liberal one into very soft stuff' is the wordplay.
'liberal' becomes 'l' (abbreviation).
'one' becomes 'i' (Roman numeral).
'into' indicates putting letters inside.
'very soft stuff' becomes 'mush' (mush means soft, wet material).
'l'+'i'='li'
'li' going into 'mush' is 'MULISH'.
(Other definitions for mulish that I've seen before include "Stubborn" , "Obstinate like a pack-animal" , "unyielding" .)