Lost heat after start of brief swim (5)
I believe the answer is:
bathe
'swim' is the definition.
(bathe is a kind of swim)
'lost heat after start of brief' is the wordplay.
'lost' is an anagram indicator.
'after' says to put letters next to each other.
'start of' says to take the initial letters.
The initial letter of 'brief' is 'b'.
'heat' anagrammed gives 'athe'.
'athe' after 'b' is 'BATHE'.
(Other definitions for bathe that I've seen before include "Wash in water" , "Steep" , "Immerse oneself in water" , "bit of swimming" , "Wash; swim" .)