Young swimmer initially drowning, passed out in story (7)
I believe the answer is:
tadpole
'young swimmer' is the definition.
(I've seen this before)
'initially drowning passed out in story' is the wordplay.
'initially' suggests taking the first letters.
'in' is an insertion indicator.
'story' becomes 'tale' ('tale' can be a synonym of 'story'**).
The first letters of 'drowning passed out' is 'dpo'.
'dpo' going within 'tale' is 'TADPOLE'.
(Other definitions for tadpole that I've seen before include "Frog in formation" , "Little creature in water" , "Young toad" , "being hatched in a puddle?" , "Aquatic larva of frog or toad" .)