Source of tuna in salad (8)
I believe the answer is:
fountain
'source' is the definition.
(thesaurus)
'of tuna in salad' is the wordplay.
'salad' is an anagram indicator (salad can refer to a mixture).
'of'+'tuna'+'in'='oftunain'
'oftunain' with letters rearranged gives 'FOUNTAIN'.
(Other definitions for fountain that I've seen before include "Fan out in ornamental water jet" , "Ornamental water feature" , "Artificially produced jet of water" , "gusher" , "Ornamental jet of water, usually in public place" .)