Fine fluffy stuff becoming very hard (5)
I believe the answer is:
flint
'very hard' is the definition.
(flint is a hard rock)
'fine fluffy stuff' is the wordplay.
'fine' becomes 'f' (abbreviation).
'fluffy stuff' becomes 'lint'.
'f'+'lint'='FLINT'
'becoming' is the link.
(Other definitions for flint that I've seen before include "Hard stone that sparks if struck against steel" , "Hard grey rock" , "Rock found in chalk" , "It is used to produce sparks" , "Hard kind of stone -- Welsh town on the Dee estuary" .)