Unique mass of pebbles that’s not hard (6)
I believe the answer is:
single
'unique' is the definition.
(I've seen this before)
'mass of pebbles that's not hard' is the wordplay.
'mass of pebbles' becomes 'shingle' (I've seen this before).
'that's not' is a deletion indicator.
'hard' becomes 'h'.
'shingle' with 'h' taken away is 'SINGLE'.
(Other definitions for single that I've seen before include "Short pop record" , "Unique" , "Sort of person" , "Just one - on one's own" , "One-way ticket" .)