Hat that's flash, worn by old sailors (8)
I believe the answer is:
tricorne
'hat' is the definition.
(I know that tricorne is a hat)
'flash worn by old sailors' is the wordplay.
'flash' becomes 'trice' (quick moment).
'worn by' is an insertion indicator (outside letters are worn by those on the inside).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'sailors' becomes 'RN' (Royal Navy).
'o'+'rn'='orn'
'trice' enclosing 'orn' is 'TRICORNE'.
'that's' is the link.
(Another definition for tricorne that I've seen is " Three-cornered hat".)