Difficult to break into small, small pieces (6)
I believe the answer is:
shards
'pieces' is the definition.
(shard is a kind of piece)
'difficult to break into small small' is the wordplay.
'difficult' becomes 'hard' ('hard' can be a synonym of 'difficult'**).
'to break into' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'small' becomes 's' (abbreviation - e.g. clothes size).
's'+'s'='ss'
'hard' inserted within 'ss' is 'SHARDS'.
(Other definitions for shards that I've seen before include "'Broken pieces, of glass say (6)'" , "Fragments of broken glass" , "Brittle fragments" , "Broken pieces of glass or pottery" , "Broken pieces of something brittle" .)