Relish (foremost of relishes) in small dish (6)
I believe the answer is:
saucer
'small dish' is the definition.
(small dish eg for a tea cup)
'relish foremost of relishes' is the wordplay.
'relish' becomes 'sauce' (relish can mean a sauce or dressing).
'foremost of' indicates taking the first letters.
The first letter of 'relishes' is 'r'.
'sauce'+'r'='SAUCER'
'in' acts as a link.
(Other definitions for saucer that I've seen before include "Piece of crockery, mysterious when flying" , "It may be flying or on the table" , "Cup-holder?" , "alien craft?" , "Piece of crockery; may fly?" .)