Toss first of seals fish (5)
I believe the answer is:
sling
'toss' is the definition.
(thesaurus)
'first of seals fish' is the wordplay.
'first of' suggests taking the first letters.
'fish' becomes 'ling' (ling is a kind of fish).
The initial letter of 'seals' is 's'.
's'+'ling'='SLING'
(Other definitions for sling that I've seen before include "Stone-throwing weapon" , "Sweetened drink with spirit base" , "Cast - used for injured arm" , "harness for broken arm" , "American drink" .)