Large vessel initially unpopular with sailors (3)
I believe the answer is:
urn
'large vessel' is the definition.
(I've seen this before)
'initially unpopular with sailors' is the wordplay.
'initially' indicates taking the first letters.
'with' means one lot of letters go next to another.
'sailors' becomes 'rn' (abbreviation for Royal Navy).
The first letter of 'unpopular' is 'u'.
'u'+'rn'='URN'
(Other definitions for urn that I've seen before include "Large vase for funeral ashes" , "Vase for containing ashes" , "Ashes trophy" , "Samovar, perhaps" , "Vessel for tea or ashes" .)