Mashed tuna initially served for relatives (5)
I believe the answer is:
aunts
'relatives' is the definition.
(aunt is a kind of relative)
'mashed tuna initially served' is the wordplay.
'mashed' is an anagram indicator.
'initially' says to take the initial letters.
The initial letter of 'served' is 's'.
'tuna' anagrammed gives 'aunt'.
'aunt'+'s'='AUNTS'
'for' is the link.
(Other definitions for aunts that I've seen before include "Sally and others" , "Elderly female relations" , "Relatives get bad US tan" , "relatively speaking" , "Relatives get a peculiar US tan" .)