Swimmer in famous commercial battle (10)
I believe the answer is:
stalingrad
'battle' is the definition.
(I've seen this before)
'swimmer in famous commercial' is the wordplay.
'swimmer' becomes 'ling' (ling is a kind of aquatic vertebrate**).
'in' is an insertion indicator.
'famous' becomes 'star' (I've seen this before).
'commercial' becomes 'ad'.
'ling' put within 'star' is 'stalingr'.
'stalingr'+'ad'='STALINGRAD'
(Other definitions for stalingrad that I've seen before include "where food was short during siege" , "Former name of Volgograd" , "Old Russian city" , "Russian scene of major World War two battle" .)