Pebbles, or just one, around harbour initially (7)
I believe the answer is:
shingle
'pebbles' is the definition.
(I've seen this before)
'one around harbour initially' is the wordplay.
'one' becomes 'single' (synonyms).
'around' indicates putting letters inside.
'harbour initially' becomes 'h' (I can't explain this - if you can you should give a lot more credence to this answer).
'single' placed around 'h' is 'SHINGLE'.
'or just' is the link.
Can you help me to learn more?
(Other definitions for shingle that I've seen before include "English (anag)" , "Mass of small rounded pebbles" , "Small beach pebbles" , "Small worn stones and pebbles on the shore" , "'hairdo'" .)