Marian periodically raised fuss about sick creature (9)
I believe the answer is:
armadillo
'creature' is the definition.
(armadillo is a kind of animal)
'marian periodically raised fuss about sick' is the wordplay.
'periodically' indicates alternate letters (letters must be selected 'every so often').
'raised' is a reversal indicator (going up in a down clue).
'fuss' becomes 'ado' (synonyms).
'about' is an insertion indicator.
'sick' becomes 'ill' (similar in meaning).
The alternating letters of 'marian' are 'mra'.
'mra' reversed gives 'arm'.
'ado' going around 'ill' is 'adillo'.
'arm'+'adillo'='ARMADILLO'
(Other definitions for armadillo that I've seen before include "Animal covered with long plates" , "9ed [SHELLED] beast" , "Mammal with protective plates" , "armour-plated type" , "Scaly mammal" .)