Large mammal is healthy when putting on weight (5)
I believe the answer is:
whale
'large mammal' is the definition.
(whales are large mammals)
'healthy when putting on weight' is the wordplay.
'healthy' becomes 'hale' (hale can mean fit and healthy).
'when putting on' says to put letters next to each other (in a down clue, some letters go on top of others).
'weight' becomes 'w' (short for weight).
'hale' after 'w' is 'WHALE'.
'is' acts as a link.
(Other definitions for whale that I've seen before include "One wallowing in the drink" , "Moby-Dick" , "Large animal" , "Huge mammal" , "Great sea mammal" .)