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