Mood sure to change in creature (8)
I believe the answer is:
dormouse
'creature' is the definition.
(dormouse is a kind of creature)
'mood sure to change' is the wordplay.
'to change' indicates anagramming the letters.
'mood'+'sure'='moodsure'
'moodsure' anagrammed gives 'DORMOUSE'.
'in' acts as a link.
(Other definitions for dormouse that I've seen before include "Alice tea party creature" , "one falling asleep over tea" , "Small rodent that hibernates in winter" , "sleepy little creature" , "Small bushy-tailed rodent" .)