Fat dog is knackered during exercise (5)
I believe the answer is:
podge
'fat' is the definition.
(I've seen this before)
'dog is knackered during exercise' is the wordplay.
'is knackered' is an anagram indicator.
'during' means one lot of letters goes inside another.
'exercise' becomes 'pe' (Physical Education).
'dog' anagrammed gives 'odg'.
'odg' going within 'pe' is 'PODGE'.
(Other definitions for podge that I've seen before include "Fatso" , "He needs exercise" , "Fat" , "One with pounds to spare" .)