Fellow having left ring on ship and dental thread (5)
I believe the answer is:
floss
'dental thread' is the definition.
(I've seen this in another clue)
'fellow having left ring on ship' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'having' is a charade indicator (letters next to each other).
'left' becomes 'l' (common abbreviation).
'ring' becomes 'o' (letter resembling a ring).
'on ship' becomes 'ss' (prefix in ship names eg SS Great Britain. I am not sure about the 'on' bit.).
'f'+'l'+'o'+'ss'='FLOSS'
'and' acts as a link.
Can you help me to learn more?
(Other definitions for floss that I've seen before include "flower in Victorian novel" , "Clean between the teeth" , "Kind of down" , "advice of dentist?" , "Eliot's river" .)