Little pebbles hard during one run (7)
I believe the answer is:
shingle
'little pebbles' is the definition.
(small stones on a beach)
'hard during one run' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'during' indicates putting letters inside.
'one run' becomes 'single' (type of run in cricket).
'h' going inside 'single' is 'SHINGLE'.
(Other definitions for shingle that I've seen before include "may be lying on the beach" , "Gravelly composition" , "Water-worn pebbles - roofing tile" , "English (anag)" , "Seashore gravel" .)