Weaklings save seconds - swapping cold for hot (7)
I believe the answer is:
shrimps
'weaklings' is the definition.
(shrimp can mean a weak or puny person)
I can't explain the rest of the clue.
(Other definitions for shrimps that I've seen before include "Small edible crustaceans" , "Sea creatures" , "Seafood; puny people" .)