Tuna swimming with small relations (5)
I believe the answer is:
aunts
'relations' is the definition.
(aunt is a kind of relation)
'tuna swimming with small' is the wordplay.
'swimming' indicates an anagram.
'with' is a charade indicator (letters next to each other).
'small' becomes 's' (abbreviation - e.g. clothes size**).
'tuna' is an anagram of 'aunt'.
'aunt'+'s'='AUNTS'
(Other definitions for aunts that I've seen before include "relatively speaking" , "Relatives get a peculiar US tan" , "Parents' sisters" , "Elderly female relations" , "Female relatives, usually older" .)